Fanon Vs Canon: Snape Edition
Snape's middle name is Tobias.
Some Potterverse characters do use their father's first name as a middle name - Harry James Potter, William Arthur Weasley - but Remus John Lupin's father's name was Lyall, according to Pottermore, and there's no canon evidence that Snape even has a middle name.
It's even possible that he has a middle name, and it's Severus. He might have a regular Muggle-type first name for use in the Muggle world (possibly even an embarrassingly old-fashioned one, such as Clarence) and Severus might have been a name which, as a child, only the few people who were in the know about magic would use - hence his smile when Lily called him by that name.
Pottermore however says that Severus Snape is his full name - no middle name.
Russian example of common toad, Bufo bufo, from the Snape hates Neville.
It is often assumed in fanfics that Snape has some kind of personal agenda against Neville. This despite the fact that although he is rude to and about Neville, that we see he only takes 3⅓ points from him and gives him two detentions (one of which is more of a covert pat on the back) in seven years, and we see him risk being sacked by Umbridge in order to protect Neville from being half-throttled. Although he's a nag to Neville, because nagging is in his nature, he is substantially less punitive towards the boy than McGonagall is, and only slightly ruder. His anger towards Neville is centred around keeping him safe, stopping him from blowing himself and his classmates up, whereas McGonagall sends the boy into a midnight wood full of giant man-eating spiders when he is only eleven, and forces him to stand outside in the corridor for weeks waiting to be let into the common-room at a time when a man she believes to be a mass-murderer has been hanging around the entrance to the common room with a big knife - yet nobody ever asks "Why does McGonagall hate Neville?"
It's certainly possible that Snape has some unpleasant associations connected to Neville, since Neville's parents were Aurors during Vold War One, and that means they were either torturers themselves, or the colleagues of torturers. Whether or not Snape or anyone he loved was ever tortured by the Longbottoms, the general fact that the Vold War One Aurors tortured suspects and threw them to the Dementors for life without a trial would have made spying!Snape's task of betraying fellow Death Eaters who trusted him even more distressing than it would otherwise have been. It's even been suggested that Snape hates Neville because if Voldemort had picked Neville as the prophecy-boy instead of Harry, Lily would not have died.
However, every negative comment we see Snape make to or about Neville is connected to his poor classroom and magical performance, so canon gives us no evidence that his nagging of Neville is anything more than a misguided effort to improve his performance, combined with irritation over Neville's continued failure to improve and perhaps coupled, latterly, with resentment over the Boggart incident. Of course, snarling and berating him is quite the wrong way to handle Neville, who needs gentle encouragement - but Snape is a very combative person himself and probably expects that Neville will work harder to prove him wrong, because that's what he'd do.
Even threatening to poison Trevor the toad isn't as unreasonable as it seems at first (quite apart from the fact that Flitwick, and Harry himself, also use Trevor to practise magic on). We know Trevor is prone to running off, and he isn't being contained, since Snape simply picks him up - he doesn't have to rummage in Neville's pockets for the toad, or demand that Neville produce him. So Neville has brought a badly-controlled, loose pet into a room full of open flames and hot and in some cases explosive chemicals - a very irresponsible thing to do, and one which Snape probably hopes to scare him out of repeating. And "poison" need not mean "deadly" or even "dangerous": any substance which does more harm than good when ingested is a poison, and it's unlikely that a third-year potion which is intended to be drunk would be terribly toxic even if it went wrong.
Nevertheless it was a cruel threat to make, since Neville himself would have feared that his own failure would cause somebody he loved to suffer or even die - something Snape ought to have empathized with. But then Snape doesn't have good rôle models. His father was angry and depressed; Slughorn is kind enough but not interested in anybody he doesn't think has great potential; Voldemort controls people by threatening their loved ones; McGonagall encourages her class to stick pins into semi-transformed hedgehogs who are still conscious and suffering; Flitwick uses Trevor to practise charms on; Dumbledore demands that Snape should pay him with a lifetime's service in return for saving the Potters. Even Hagrid is only kind to animals, including humans, if he happens to like them. Brutal insensitivity is a cultural norm.
Blainville's horned lizard a.k.a. horned toad or horny toad, Phrynosoma coronatum, from California Herps, photo' by Gary Nafis. There are several species of "horned toad" which vary slightly in coat pattern, spike formation, tail-shape and degree of flatness
There's an odd bit of business where Rowling may well have intended Snape to be being spiteful to Neville, but if so it hasn't turned out that way. That we see, Snape only sets Neville two detentions in seven years, one of which is the one in seventh year where he sends Ginny, Luna and Neville to Hagrid to protect them from the Carrows, and then puts it about that they've been cruelly punished - although clearly this is not so much a punishment as a covert pat on the back. The other occasion is in fourth year, after Neville has melted his sixth cauldron, when Snape sets him to disembowel a barrelful of horned toads - a spiky, flattish kind of lizard with only the most cursory resemblance to a real toad. It seems likely that Rowling intended Snape to have set a boy who loves toads to dissecting toads -
Surinam horned frog, Ceratophrys cornuta, sometimes miscalled a horned toad, from Wikipedia: Surinam horned frog, photo' by Maarten Sepp
especially as Harry thinks that Neville has "frog guts" on his hands - but whatever the intention it says right there in the text that Snape made Neville dissect lizards, so Rowling's putative intention clashes with canon.
There does exist a creature called a Surinam horned frog which is occasionally miscalled a horned toad, and given Harry's reference to frog guts you could say that that was what Snape had Neville dissect. But even though they're both anurans, the horned frog doesn't look much more like Trevor than the horned lizard does. It's a weird thing with a head like an upturned bucket, and resembles a common toad only about as closely as an orang utan resembles a human.
You could say that making a sensitive boy like Neville dissect any animal is cruel - but not more so than forcing him to Transfigure living animals into pincushions and then stick pins in them to see if they still flinch, or ordering Hermione, who loves cats, to Vanish kittens "into non-being", both of which we see McGonagall do without adverse comment. Extreme callousness towards non-human animals is a Hogwarts norm, and at least Snape did not, so far as we know (and unlike McGonagall), require Neville to hurt or kill the animals he was going to cut up.
We know Neville has some sort of disability of memory. He forgets passwords, his family sends him a Remembrall - he could have Attention Deficit, or perhaps he is dyslexic and cannot follow the written instructions on the board. His poor memory may be the direct result of the fact that his uncle used to put him into repeated danger of death in an attempt to squeeze more magic out of him, as severe prolonged childhood stress due to abuse is known to damage the hippocampus, causing memory problems.
The wizarding world, however, seems to have no awareness of learning difficulties, and McGonagall too is very hard on Neville as a result. On top of this, Snape must be frightened all the time Neville is brewing, which isn't a good state from which to be lovingly patient, especially if you're Snape and weren't very patient to begin with. Snape, after all, is shut in a small room with somebody who is prone to causing accidents involving fire and potential explosives, and who may at any moment kill or severely injure him, themselves and/or one or more classmates.
Snape is probably baffled and frustrated by Neville's poor performance, when he is known to be a star Herbology student and as such must be quite bright, and when Herbology and Potions are so closely allied. We never see Snape criticise Neville's written work. He seems to cope better with Crabbe and Goyle, who are genuinely thick - he snaps at them, gives Crabbe lines to write and tries to get them to pull their metaphorical socks up, but he doesn't get as excited about them as he does about Neville.
It is conceivable that he is right to be baffled and that on some level, whether consciously or unconsciously, Neville is failing deliberately. It seems likely that his grandmother will try to pressurise him into becoming an Auror like his parents, whether he wants to or not, and failing at Potions would be a surefire way to render that impossible. It is even conceivable, although a bit of a stretch, that Snape deliberately mismanages Neville in an attempt to make him do poorly in Potions, because he's reasoned that if Neville gets into NEWT Potions, and passes, his gran will force him to become an Auror and the boy will end up dead. It would be a rather cruel way to protect Neville, but Snape has an established pattern of caring very deeply about his students' physical safety, and very little about their happiness.
It has also been pointed out that Neville's performance seems to improve after his father's wand is broken at the end of fifth year, and he finally gets a fresh one of his own.
Even though in canon Snape is less punitive towards Neville than McGonagall is, the fact that it is Snape who is Neville's Boggart, rather than the Death Eaters who tortured Neville's parents into madness, leads fanon to assume that Snape must be horribly cruel to Neville, even though all that we are shown is a rude overbearing ill-tempered nag who nevertheless almost never actually punishes the boy, and criticises his performance rather than his person; and even though Neville only interacts with Snape for three hours a week of which Snape spends perhaps ten minutes a week nagging him, and Neville does not otherwise come across as an especially timid or fragile person. But there are several possible reasons why Snape might be Neville's Boggart, without having done anything very dreadful to him.
The first and most obvious one is that Neville, with his family connections to the Aurory, knows that Snape is a former Death Eater, and this would make sense in terms of the structure of the story. In PoA nearly everybody (including Harry, right up until the Shrieking Shack scene in mid June) believes that an ally, Sirius, is a murderous Death Eater who has come to Hogwarts to kill Harry. In GoF nearly everybody (including Harry, right up until the graveyard scene in mid June) believes that a murderous Death Eater who has come to Hogwarts to kill Harry is an ally, so that the two books in some sense form a mirror-image pair. It would make sense thematically if Neville's fear of Snape is intended to foreshadow the revelation of his Dark Mark in GoF.
Then, Neville's family put him in danger of death to try to squeeze more magic out of him, and Snape constantly reminds Neville that he is bad at magic, so Snape may make him think about danger and rejection by his family. Duj suggests that even though as at the date of the Boggart lesson Snape has never, that we have been shown, actually punished Neville, just nagged and criticized him, Neville has been brought up with people who subjected him to life-threatening violence when he was bad at magic, so he may expect Snape physically to attack him, despite all the evidence to the contrary. It may be significant that the Snape-Boggart "... was bearing down upon him, reaching inside his robes", as if to draw a wand, supporting the idea that Neville expects Snape to attack him, either because he expects to be physically punished for beibng bad at magic, or because he expects the ex Death Eater ro carry out an attack.
Nightfall Rising has pointed out that there is evidently some physical resemblance between Snape and Neville's grandmother, and the Longbottoms are supposedly a Yorkshire family - they certainly have a Yorkshire name - and Snape, although Pottermore places him in the Midlands, comes from a place which looks like it's probably as far north as the Midlands can get, so there may be some similarity of accent too. So Snape may trigger fears which in fact revolve around Augusta.
Then there's Snape's generally sinister, Goth appearance, which may have caused Neville to develop a phobia about him out of proportion to his actual behaviour, and the fact that the session with the Boggart took place only a couple of hours after Snape had threatened to poison Trevor, and immediately after Neville had seen Snape again in the staff common room and Snape had been rude about him to Remus, so Snape was on his mind that day.
Finally, Neville may be scared of the subject itself - of the fires and the explosions, but also of the fact that Potions is one of the subjects his gran wants him to succeed at so he can be an Auror like his dad.
You also have to wonder why Neville would be so nervous of Snape, who snarls at and verbally bullies him but also physically protects him, and not apparently scared of McGonagall, who puts him in serious danger. But given what we've been told about Neville's family, he may regard being put into severe danger by somebody who is supposed to be caring for and teaching him as normal. He may even see it as a proof of caring, as women in some rough areas often used to think that if their man didn't beat them up he must not be passionate enough about them: the fact that Snape tries to keep him safe may be part of what makes Neville uneasy about the man, because it feels unnatural and disturbing to him.
Interestingly, when Snape warns Lockhart not to use Neville to demonstrate duelling techniques because "Longbottom causes devastation with the simplest spells, we'll be sending what's left of Finch-Fletchley up to the hospital wing in a matchbox", he's suggesting that Neville - who believes himself to be almost a Squib - has a great deal of magical force, even if he can't control it. If this is accurate - if he isn't just saying this off the top of his head - it suggests Snape has taken an interest in Neville outwith Potions class, since Neville's spell-work doesn't come up in Potions, so far as we know.
Snape's relationship with Lily is stalkerish.
In the playground scene young Snape is described as watching Lily in an almost greedy way, but his greed cannot have been sexual in character, unless he was an incredibly early starter, for he was a pre-pubertal child of around nine at the time. Eleven-year-old Harry is also described as staring at Lily in the mirror "hungrily", and nobody assumes this means he wanted to have sex with her.
Snape's nine-year-old passion for Lily was of a piece with - and very possibly inspired by - Charlie Brown's fascination with his little red-headed girl. So young Sev's greed must have been for something else - for the magical power Lily displayed, for the chance to have someone his own age with whom he could share the magical side of his being without being thought a freak, for friendship, for confirmation that the magic world existed outside his own family, for a bit of colour and beauty in his mostly-depressing life. She then went on to be one of the few people that we know of in his life who was ever kind to him, at least some of the time, and we see from his devotion to Dumbledore that Snape will probably be loyal to anyone who shows him affection, even intermittently.
[It may well be that that was how Tom Riddle recruited Snape. We see from the way he charmed the information about Horcuxes out of Horace Slughorn that Tom was very good at putting on fake kindness, fake affection, to win people over; and the fact that he apologized to Snape before killing him suggests that Tom may genuinely have felt as much real fondness for Snape as he was capable of feeling for anyone.]
Since we can see that young Severus was already devoted to Lily before he was old enough for that devotion to have a sexual element, there's no reason to think that his teenage and adult devotion to her was wholly or mainly sexual either. Sociological studies in Israel many years ago indicated that on hard-line kibbutzim where the children all slept at night in a communal Children's House, those children were unlikely to develop sexual feelings for each other in later life, even when they were totally unrelated, because their early experience had caused them to see their house-mates as siblings, not romantic propects. It's highly possible that two children who had been close friends since they were nine would also see each other as brother and sister, and thematically that would tie Snape's undying devotion to Lily in with Dumbledore's century-long grief for Ariana.
At the same time, the courtyard scene does suggest that Snape's teenage love for Lily might have been at least partly sexual. His relief at discovering that Lily still disliked James must have been partly because she wasn't about to put herself in danger from Sirius and Remus, but his floating-on-Cloud-Nine happiness does suggest that there was more to it than that. However, his devotion predated his capacity for sexual preferences, and there's no evidence that that platonic devotion ever went away, whatever else may have been laid on top of it. Hence, there's no canonical reason to think that his long-term dedication to Lily's memory was primarily sexual in nature, although it's just about canon-compatible on a longish hook.
We know he gave Voldemort the impression that his interest in Lily as an adult was primarily sexual possessiveness, but nothing in canon suggests that he was telling the truth. Indeed the way he seems to make Lily the lodestar of his life suggests that even his sexual interest in her tended towards a Mediaeval style of formal "courtly love", where the desired girl becomes a sort of idealised personal goddess or private monarch rather than a sex object.
Even the scene in the courtyard where young Snape says to Lily "I won't let you ..." follows on from the abortive conversation about what Remus is, and so is probably protective rather than controlling. In context it seems likely that what he was trying to say was that he wouldn't let Lily sleepwalk blind into a closer relationship with a gang which she didn't know included a werewolf and a would-be murderer, but he then realised he couldn't explain what he meant without breaking his promise not to out Remus.
We see, in that scene, that Snape is still devoted to Lily but she seems to be cooling towards him (she shows no concern about the fact that he has had a life-threatening experience since the last time she spoke to him, but launches straight into criticising him). However, his continued affection cannot be said to be stalkerish because she encourages him in it, reassuring him that they are still best friends even though it seems unlikely that this is still the case from her point of view.
We know he continued to carry a torch for her and hope for a reunion after she had broken off their friendship, because Rowling has said that he joined the Death Eaters in part because he hoped to impress Lily - but this is not unreasonable given that she had started dating a bully who had publicly threatened her with violence, suggesting that she might be turned on by thugs. We don't know whether Snape showed any unreasonable expectations of an adult relationship with Lily, or made a nuisance of himself to her in any way - it's not inconceivable that he might have done, but there's no canon evidence for it.
And far from being controlling towards her, he seems to be as devoted as a puppy. It's true he seems quite blind to her upset after her argument with Petunia - but then Lily seems to get angry and blame him for things which aren't really his fault quite a lot and he just accepts it. You could say he isn't taking her emotions seriously - or you could say he's being tolerant and accepting of her combative personality - or you could say that life has already taught him to expect to be kicked in the teeth even by loved ones.
In any case Snape's presumed-to-be-at-least-partly-sexual feelings for Lily seem less extreme and dangerous than Harry's feelings for Ginny, which cause him to feel as if jealousy is a raging monster trying to break out of his ribcage.
I gather some fen have accused Snape of suffering from Nice Guy Syndrome, where a man cultivates a friendship with a woman in the hopes of getting to sleep with her, and if she doesn't he attributes this to his being too "nice" and says that women evidently prefer brutes. To begin with the way it's described on the net all sounds terribly American: here in the UK guys who are a bit whiny and needy will probably still get laid so long as they're not violently revolting, because we don't really have the same construct of masculinity. But in any case Severus was genuinely Lily's friend since well before sex could have been an issue; he is reassured by her that he is her best friend; that we see he doesn't ever try to press her for sex or act as if he thinks he is entitled to her, he just lusts wistfully from afar; and considering the way we see James treat Lily while trying to "win" her, and yet she takes up with him anyway, it seems she genuinely does prefer brutes.
In fact it is James who shows some of the "Nice Guy" symptoms. Interview canon is that James persecuted Severus because he was jealous over Lily, so even though she wasn't his girlfriend, or for that matter Sev's girlfriend, and for most of their time at Hogwarts she had shown no interest in him, James was still so jealous over and possessive of Lily that he couldn't stand the idea of her spending time with another male, and spent seven years persecuting another boy to punish him for having dared to have met Lily first. He acted as if his status as Quidditch Jock should entitle him to Lily's favours if he wanted them, and was prepared to use underhand tactics including emotional blackmail and bullying in an attempt to force Lily to date him - we see him attack her friend and then offer to stop persecuting him if Lily will go out with him, without apparently caring in the slightest whether it's what she herself wants or not. When she protested about the attack on her friend and tried to stop it, James threatened her with violence. Later he successfully persuaded her to go out with him, not primarily by reforming his behaviour but by getting better at lying to her to conceal it. According to Pottermore, once they were dating he was obnoxious to her family and drove a wedge between her and her sister and brother-in-law, by being obnoxious to them and bragging to them about how much richer than them he was - although he was provoked, in this case, by Vernon's assumption that he was unemployed because he was lazy, rather than because he was independently wealthy.
Snape, so far as we know from canon, never even asked Lily to go out with him, far less tried to force her to against her will. That we know of, the only thing he ever pressed her to do was to accept his humble apology, and so far as we know he accepted her refusal and never bothered her again. JKR has said that he joined the Death Eaters in part because he hoped to impress Lily, so he was still in some sense "carrying a torch" for her, but Sirius and Remus seem unaware of the strength of his feelings for Lily, which is strong evidence that he left her alone and didn't try to get her back once she'd taken up with James.
That he continues to be so devoted to Lily's memory may seem a bit obsessive but he is still only in his thirties when he dies, and Lily's death is less than seventeen years ago. It is less than ten years ago when we first meet him. In any case infinitely extended, devoted teenage love and undying devotion to a dead loved-one are two of JKR's favourite tropes - possibly because it means she doesn't have to write the older characters a current love-interest.
Dumbledore is portrayed as still so obsessed by his sister's death that nearly a century later he puts on a cursed ring which ultimately precipitates his own death, just in the hopes of being able to see her again and apologize for his role in her untimely demise; and Rowling has said that he Never Loved Again after Grindelwald. Sirius is still so devoted to James fifteen years after his death that he tries to make Harry be James redux.
On Pottermore we're told that Minerva McGonagall fell madly in love with a Muggle neighbour named Dougal McGregor in summer 1954 when she was eighteen, then jilted him and broke both their hearts because she didn't want to have to give up magic to live in the Muggle world. She worked for two years at the Ministry before starting teaching at Hogwarts in December 1956, but continued to carry a torch for Dougal (even after he married someone else) and Could Love No Other for twenty-eight years - despite repeated proposals of marriage from her former boss at the Ministry, Elphinstone Urquart - until 1982, when she heard that Dougal had died, and promptly married Elphinstone. He died in an accident three years later, leaving her to a state of perpetual widowhood, even though at fifty she was not all that old for a witch.
And almost everybody in the Potter books, if they marry at all, marries somebody they already knew and probably fancied at school. James and Lily, Molly and Arthur, Harry and Ginny, Ron and Hermione, Hannah and Neville, Draco and Astoria, Bellatrix and Rudolphus, probably Lucius and Narcissa and Andromeda and Ted (because if you do the maths Andromeda probably fell pregnant with Tonks immediately after leaving Hogwarts, if not before).... So even if you think Snape's behaviour is obsessive, it seems to be pretty much a cultural norm for his society.
Snape ignored a live, injured baby Harry to cradle Lily's corpse.
There is a scene in the films - and only in the films - where Snape is at the Potters' house at Godric's Hollow after the attack, cradling Lily's dead body in his arms, crying, and ignoring baby Harry and his cut face. This is widely held up as proof that his love for Lily was obsessive and that he was callous towards baby Harry. But aside from the fact that in the books he was in Dumbledore's office at Hogwarts at this point, even within the film universe his action was not unreasonable. Since Avada Kedavra leaves no marks, he couldn't know Lily was dead until he had examined her, and if she had been alive but unconscious her medical condition would probably have been much more urgent than Harry's. That's if he could even see Harry. The Fidelius protected the knowledge that the Potters lived at that house, and we're told that you could look through the window at them and not see them, so the charm concealed the Potters but not the house. We know Snape hadn't been let inside the Secret, because if he had been he would have known Peter was the traitor. Whether the Fidelius was still in place or not, it wouldn't stop him seeing Lily, because she no longer lived at that house; but if the Fidelius was still active, he wouldn't be able to see Harry, living in the ruins.
Snape was part of Voldemort's inner circle.
There's a common fanon idea that Snape is a Big Name Death Eater, part of Voldemort's inner circle. This has a certain amount of canon support in that Harry thinks, in DH, that "The werewolf might be allowed to wear Death Eater robes when they wanted to use him, but only Voldemort’s inner circle were branded with the Dark Mark: Greyback had not been granted this highest honour." And yes, Snape had the Dark Mark (although we don't know whether he was Marked before or after he defected, and Dumbledore started feeding him tidbits of information to give to Voldemort). But so did bottom-feeders like the Carrows, and sixteen-year-old Draco, so as inner circles go the circle of the Marked wasn't exactly an exclusive club.
As far as Snape belonging to some more significant inner group goes, by Vold War Two there were only a few Death Eaters, and Snape was picked to become Headmaster, so it's reasonable to assume that at that point he is one of the top players. But during Vold War One there were around four hundred Death Eaters (Remus says they outnumbered the Order twenty to one)and Snape, when he asked for Lily's life, was only twenty. We also know that he had no reputation as a Death Eater at that time, since Sirius, an Order member, had not heard any rumour that he was one. It seems unlikely that he could have been in any sort of serious inner circle at that age.
So why did Voldemort try to spare Lily to please him? When Voldemort kills Snape, he tells him that he regrets it, which suggests that he is as near to being fond of Snape as he is capable of being. Perhaps he always had some fellow-feeling for a dirt-poor half-blood, having been one himself. We also have Rowling's statement that Voldemort had previously tried to recruit Lily, so he must have valued her, and perhaps hoped to try again.
Snape committed atrocities when he was a Death Eater.
This one is just about canon-compatible, but it's a stretch. The evidence for it is just the fact that he belonged - probably fairly briefly - to a terrorist organisation which certainly carried out a substantial number of killings and torturings of political targets (although the extent to which they may or may not have killed or tortured just for the sake of it is moot), the fact that Amycus, Alecto and Greyback seem very respectful of him in the Tower scene, and Rowling's statement that he had been "drawn to loathesome people and acts". On the other hand, there's quite a lot of evidence against it.
Firstly, there is the fact that even Sirius, who was an Order member and who hated Snape, said that in Vold War One there had been no rumour that Snape was a Death Eater. We know that he was - but this suggests that he didn't do anything which attracted attention. Then there's the fact that later, in HBP, Bellatrix accuses him of being, as a Death Eater, all talk and no action.
Then there's the argument he has with Dumbledore about Harry's fate. Dumbledore is on the back foot, being defensive and looking for a weapon with which to defend himself, yet he does not ask Snape how many people he has killed - only how many he has seen die (and Snape says latterly, only those he could not save). And Snape expresses concern as to the possible damage to his soul if he kills Dumbledore, with no suggestion from either of them that he has killed before.
Talking to the young Tom about Horcruxes, Slughorn says that the soul can be split "By committing murder. Killing rips the soul apart." This is slightly ambiguous, but there seems to be no suggestion that Molly's soul will be split by having killed Bellatrix, although she certainly will not feel any remorse, so that tends to confirm that it is murder specifically which splits the soul. Dumbledore says that only Snape himself can tell whether carrying out a mercy killing will damage his soul, and does not seem to be saying "It will be damaged but your remorse will heal you", which again hints that Dumbledore does not think that any kind of killing necessarily splits the soul, only murder (and whether Snape's mercy-killing of Dumbledore will count as murder or not will depend on what's in his heart when he does it). So if your fanfic requires it you can have Snape having killed in battle or as an act of mercy while he was a Death Eater, without raising canon issues about his soul.
When Rowling was asked at interview - presumably before HBP came out - whether Snape could see Thestrals, she replied that he could, because as a Death Eater he would have witnessed "things". There was no suggestion that he had killed anyone himself, or taken an active part in the "things", and she went on to soften it further by saying that most of the other staff could see Thestrals too. She has also said that he joined the Death Eaters because he felt isolated and wanted the security of belonging to a group, and because he hoped it would impress Lily - no mention of political fanaticism or wanting an opportunity for violence.
And whilst we know that he did relay the first half of the prophecy, there was nothing in the half he heard to indicate that "the one" was a still-to-be-born baby, or that it was male - or even that it was a person rather than a weapon of some kind, and that the word was "born" rather than "borne". It could have meant that a powerful weapon was being conveyed ("borne") to some people who had thrice defied Voldemort. All he could say was that if it referred to a person, and the word was "born", then it must be somebody young enough to have parents who were still alive at some point after Voldemort's rise.
Then there's his own nature, which seems to be verbally aggressive, but physically gentle. As a boy he dropped a branch on Tuney, probably subconsciously (just like Harry inflating Aunt Marge), but as an adult he is remarkably non-violent. When he believes that Sirius is a mass-murderer who has come to Hogwarts to kill Harry, he wants to destroy him, to feed him to the Dementors: but still when he has to carry Sirius unconscious, he uses a stretcher. The idea of seizing the opportunity to knock his unconscious enemy around, as Sirius had done to him, evidently doesn't occur to him.
When he has the chance to hurt Peter Pettigrew, the man who bullied him at school and then betrayed Lily to her death, slaughtered twelve Muggles and condemned Sirius to Azkaban for a crime he hadn't committed, so far as we see Snape just sneers at him a bit and makes him do a bit of fetching and carrying about the house, and stings him with a minor hex to stop him eavesdropping (possibly a reference to the fact that John Nettleship's nickname was "Stinger" - the stinging nettle being one of our commonest British plants). When Harry calls Snape a coward, and he is in emotional agony, he gives him a magical slap - in contrast with Remus, who threw Harry into a wall when Harry called him a coward. He allows Buckbeak to tear at him, and does not try to defend himself. He uses only defensive spells when McGonagall, Flitwick and Sprout are doing their best to harm him.
We are told that latterly Snape saves people from the Death Eaters whenever possible, even though that doesn't really serve either Lily's or the Order's interests. OK, it's Snape who says so, but we have other evidence that his urge to save people is powerful - we see him risk blowing his cover during the Polyjuice chase in order to protect Remus, despite Dumbledore having told him to keep his head down. He risks antagonising Umbridge to prevent Neville being half-throttled, and does his best to warn Sirius not to go to the Ministry. He clutches the back of a chair when he hears that Ginny has been taken into the Chamber, and turns white-faced and angry when he sees the memory of Cedric's death. He sprints through the school in his nightshirt because he heard somebody scream, and runs through a bathroom door, ashen-faced and without checking what's on the other side first, because a girl's voice cried "Murder".
Even Sectumsempra, which he either invented or learned especially, seems to have strict limits. Harry strikes at Draco with Sectumsempra and cuts him quite badly, but he doesn't carve him open or chop him in half, as would happen if he'd hit him with a real sword. He hacks at the Inferi with all his might, and he cuts through their wet clothes which shows how sharp the magical blade is, but he still only inflicts shallow wounds. It looks as though the spell is rigged not to cut too deeply, or not to cut bone (although it does cut ear-cartilege).
The only time we see adult Snape be physically violent, outside of the Duelling Club, is when he hauls Harry away from the Pensieve and throws a jar at him - and he misses, probably deliberately.
Altogether, despite his in some ways aggressive nature, Snape seems to have a strong urge not to hurt people physically, at least when he's not in a combat situation. Physical violence just seems to go against the grain, for him, so it seems unlikely he would have been very violent even as a Death Eater, if he could get out of it.
A prominent fan-writer has suggested that young Sev must have been required to carry out atrocities as part of his induction into the Death Eaters - but there's no canon evidence for this, and I've never heard of our home-grown British terrorist organizations doing anything of the sort.
The extent to which young Snape bought into the Death Eaters' ethnic theories at the time is uncertain - quite apart from the fact that Rowling says that Voldemort tried to recruit Muggle-born Lily, showing that the Death Eaters were not as racist (or magicist or whatever you want to call it) in Vold War One as they would be once Umbridge got involved. He says that he doesn't want to speak to Petunia because she's a Muggle - but that's reasonable, because the reason he wants to speak to Lily is because he wants someone to share magic with, so this is like one musically-gifted child seeking out another, and not wanting to talk to their sibling who is tone-deaf. Later on the train he says that Tuney is "Only a..." and the next word might have been going to be "Muggle" or it could have been "snidey little cow", we don't know. But even if it was going to be "Muggle", nobody seems worried by the fact that Hagrid uses "Muggle" to mean "knuckle-dragger".
Severus apparently uses "Mudblood" freely in fifth year, but Lily has evidently tolerated this up until the underpants incident, and since he's a half-blood in Slytherin himself his use may have been ironic or self-protective. He calls Lily a "filthy little Mudblood" when he is angry and desperate, but there are several questions here which bear on the extent to which he might have been truly bigoted and might truly have bought into an anti-Muggle or anti-Muggle-born agenda.
We don't know whether he's saying Lily is filthy because she's a Muggle-born, whether he's implying that her blood is literally dirty - but he probably isn't. We occasionally see it used that way by other characters, but in most cases where characters in the Potter books - or Rowling herself - use "filthy" as a form of profanity, it's a general emphatic like "bloody" or "blasted". That's probably how Snape himself uses it, since he also applies it to pure-blooded James ("Your filthy father"), so you have to think of it as if he had said "bloody little Mudblood".
Lily and Severus have been drifting apart for some time. We see in the courtyard scene, a few days after the werewolf incident, that Lily knows Severus has had a life-threatening experience from which he had to be rescued, and this is the first time she's spoken to him since, but she isn't interested in how Severus is or whether he was frightened, just in criticizing him and his choice in male friends. And although he's meant to be her friend she arrives already believing James's version of events and won't give any credence to Severus - even though it's Severus who is telling the truth. So he already has good reason to be unsure of her.
Now Lily has just suppressed a smile on seeing Severus's bare legs exposed. She probably means it affectionately but he is very raw and stressed and if he sees it he probably thinks she's jeering at him - especially as she later does just that, calling him "Snivellus" and sneering at his poor clothes, so it's not an unreasonable thing for him to expect. We've heard what was probably her laughing at him as he struggled with his broom, after all. And Rowling has said that in the underpants scene, Lily was actually flirting with James, or at least already fancied him, and a future spy and Legilimens would probably be very good at reading body-language, so Severus would be aware of Unresolved Sexual Tension between James and Lily, and be jealous and upset that his misery was being made part of their courtship. Also, as a dirt-poor, working-class half-blood in Slytherin, allowing himself to be seen to be rescued by a Muggle-born Gryffindor girl could probably have got him into serious trouble with people who could get at him while he was sleeping.
Then, he knows, although he isn't allowed to say so, that Remus is a werewolf. He may be raging at Lily because she is putting herself in danger by going near the Marauders, in the way that parents rage at a child who has run out into traffic, and want to drive her away from James who has just threatened to hex her. He may also want Lily to be (temporarily) angry with him, because he's just heard James offer to stop persecuting him, Severus if Lily will go out with him, James, and Sev would rather be persecuted than see Lily dating James, so he doesn't want her to feel like sacrificing herself by dating James to save him.
Also, the spell which James uses to suspend Severus in the air, Levicorpus, is one of Sev's own devising. We're told that it enjoyed a vogue during fifth year, and presumably that was before this scene which took place within two or three weeks of the end of the academic year. When Harry talks to Remus about it it doesn't really sound as if Remus already knew this was one of Snape's own spells, so it's unlikely young Severus had openly revealed this spell as one of his, and it's non-verbal so it wouldn't have got about through people hearing him use it. Wolfwillow has suggested that young Severus didn't know how this spell had got into other people's hands, but that he had (almost inevitably, especially if what he was initially trying to do was to imitate her flying from the swing) shared it with Lily, and then suspected her of sharing it with fellow Gryffindors without permission.
In fact, James could and probably would have got it by spying on Sev and Lily from under his Cloak, while trying to decide how best to ask her out, but at that time Sev didn't know James had a perfect Invisibility Cloak, so when James turned his own spell on him that would make him furious with Lily for having (as he thought) been the one to put that spell in James's hands. That would then feed adult Snape's anger at the end of HBP when James's son tried to use the same spell against him, since by that point he knew about the Cloak and the Map and knew that because of James's sneaking about he had falsely accused Lily, and it was partly because of that that he had lost her friendship.
So there are many complex reasons why he might in that moment have wanted to offend Lily by using an offensive word, without neccessarily genuinely buying into the racist theories that underpinned it. The fact that he called himself the Half-Blood Prince rather sounds as if far from being a pure-blood supremacist, he gloried in his own mixed-blood status and wanted to rub his snooty Prince relatives' noses in it.
It's unpleasant of him to use a racially-offensive term, of course it is, but he's half-Muggle himself so to some extent he has rights over offensive terms used of people with Muggle blood, just as black people have rights over offensive terms used about black people. And in this group, at least, even if probably not in the Slug Club, Snape is an isolated outsider while Lily is popular and surrounded by allies and admirers, and he is very poor while she is, if perhaps not quite middle class, of a social level that looks down on people from Spinner's End as if they were dirt. Some people argue seriously that it is impossible for the disadvantaged to be racist towards the advantaged, that racism by definition is something the strong do to the weak. Personally I think that's a ridiculous argument, it's just re-defining words to suit your own agenda, but to anyone who actually believes that argument, Snape cannot be being racist here, because he is in a weak social position and Lily is, despite being Muggleborn, in a strong one.
I see some of the Snape-haters on the net have been screaming that I am an awful person because I have suggested that young Snape could be anything less than totally evil even though he used a racist word. This just goes to support my thesis that most Snape-haters are very young, because they clearly have no idea what the 1970s were like or what was socially acceptable then. And, as always, they never seem to worry about the almost universally patronising attitude of wizards towards Muggles or the fact that Hagrid clearly uses "Muggle" as a term of racial abuse - perhaps because they themselves don't regard a half-giant as a fully sentient person with responsibilities, or because they think it's OK to discriminate against ordinary human beings for being ordinary.
If Snape - or indeed Lily or Petunia - was racist it would not be very surprising, however. Pottermore canon says that Cokeworth, their home town or suburb, is in the Midlands and the Midlands in the 1960s were a hotbed of racism in the Muggle sense. They would have heard adults around them, perhaps including their own parents, casually employing racist language throughout their childhoods, and it's to Petunia's credit that she admires Kingsley rather than being prejudiced against him.
Note that we do not know when Snape joined the Death Eaters, except that it was probably after the underpants incident in June 1976 (at least, if he was already a Death Eater at that point James didn't know it, since the greatest crime he accused Snape of was existing), and presumably before Trelawney made her big prophecy, which happened at the very end of 1979 or start of 1980. Draco apparently was Marked as a Death Eater within a month or two of turning sixteen (his birthday is on 5th June and he was showing his Mark in Borgin and Burkes a few days into August), so it is certainly possible that Snape joined as early as 1976. He defected to the Order side late in 1980 or early in 1981. The longest period for which he could have been a genuine Death Eater, then, is just over four and a half years and the shortest just under a year.
There is, however, one very small suggestion that he became a Death Eater around the middle of 1979, meaning that he was a true Death Eater for around eighteen months. In the Skinner's End scene, he tells Bellatrix that when Voldemort returned to life and summoned him - that is, in June 1995 - he had sixteen years' worth of information about Dumbledore to offer him. This doesn't make much sense however you cut it, since Voldemort had only been trapped in half-life for fourteen years, and at the point when he was disembodied he had presumably been up to date on whatever information Snape was willing to give him. Either Snape is as bad at maths as his author - which is unlikely in somebody who has to work out precise combinations of ingredients without a computer, and hasn't yet blown himself up - or his mind is dwelling on the date on which he first began to collect information for Voldemort, sixteen years prior to summer 1995.
There is also the point that in late 1979 or very early 1980 Dumbledore allows young Snape to leave The Hog's Head after overhearing Trelawney's prophecy, without Obliviating him. This seems a huge oversight and plot hole, unless Dumbledore actively wants Voldemort to hear the prophecy, or he does not know Snape is a Death Eater. If he does not know that Snape is a Death Eater, either his intelligence-gathering is poor, or Snape hasn't been one for very long - or perhaps isn't one yet. You could make a case for Snape overhearing the prophecy in late 1979, taking the warning to Voldemort and only joining the Death Eaters at that point, having been undecided up to then. It would explain his emphasis on his usefulness to Voldemort being the information he had about Dumbledore, even in 1979.
Snape was Voldemort's potioneer during Vold War One.
It tends to be taken as a given (especially by fan-writers who have noticed that the evidence suggests that Snape wasn't very visible as a Death Eater) that the main service Snape rendered to Voldemort before he became a teacher was to brew new and deadly potions. On one level this would make perfect sense.
However, we have to remember that Snape originally applied to Dumbledore for the DADA post, and whatever Voldemort expected the outcome to be, Snape would have to seem like a reasonable candidate if his application was not to rouse Dumbledore's suspicions. This quite strongly suggests that he had spent the intervening eighteen months since leaving school (he finished school in summer 1978 and internal evidence, below, suggests that he made his initial job application for the DADA post very early in 1980) working at something which would fit him to teach Defence. The most probable option seems to me to be that he was a curse-breaker or some sort of magical bodyguard, but it's not inconceivable that he started Auror training and then quit to join the Death Eaters after seeing how cruel the Aurors had become (and not initially realising that the Death Eaters were even worse).
We know Snape started teaching in autumn 1981 (in autumn 1995 he tells Umbridge he has been teaching at Hogwarts for fourteen years). It's unclear what he did between early 1980 and autumn 1981 when he was given the Potions post. It could be that he was given some other job at Hogwarts, perhaps as a teacher's assistant, which would explain why he tells Bellatrix that at the time of Voldemort's reconstitution, in summer 1995, he had sixteen years' worth of information about Dumbledore to give him.
But then, we see him defect in winter and he cannot very well have defected before Harry was born - before the Dark Lord could know that Harry would be born as the seventh month dies. So his defection must have been in winter 1980/81. If he had been working at Hogwarts, why would he defect on a bare hillside instead of in an office? And it's clear Dumbledore knew he was a Death Eater at that point, and it doesn't seem likely he would already have employed him on those terms. So Snape applied for the DADA post and heard the prophecy early in 1980, defected in winter 1980/81 and started teaching at Hogwarts after his defection and probably in autumn 1981, so if you need him to be Voldie's potioneer full time there's an eighteen month gap where it could fit, between early 1980 and autumn 1981. And of course, he could have been a curse-breaker and yet also made potions for Voldemort in his spare time.
Snape defected just before the end of Vold War One.
This one is the result of confused and conflicting information in the books, and is just about possible, but unlikely. Whenever you think he defected, there's some piece of confusing information which needs to be explained, because however you cut it there's a socking-great continuity error involved.
We are told that the Potters went into deep hiding under the Fidelius charm about a week before their death - i.e. around 24th October 1981. This quite naturally leads people to suppose that Snape only defected, and warned them that Voldemort was targetting them, round about 20th October. However, there are a number of problems with this idea.
The first issue is that Lily wrote a letter which appears to have been written quite soon after Harry's first birthday party, and very soon after the deaths of the Mackinnons (which she refers to as if it was recent, shocking news), and in it she says that "James is getting a bit frustrated shut up here", especially as Dumbledore "still" has his Invisibility Cloak, without which he is apparently unable to go out. Clearly, they are already in hiding. The fact that the Dumbledore Harry meets at the astral King's Cross says that he borrowed the Invisibility Cloak only a few days before Lily and James died supports the idea that Lily's letter was written three months after Harry's first birthday and a day or two before she died, despite sounding as if his birthday party was a recent event.
However, the first problem with this is that Lily says that "Bathilda [Bagshot] drops in most days", so if this really was written just before the Potters died they must have allowed Bathilda inside the Fidelius, which doesn't sound as if they are maintaining a panicky high security which has only been in place for a few days. And it seems unlikely that even James would be all that frustrated if he had only been in hiding for six days - or that either Lily or James would be complaining that Dumbledore "still" had the Cloak if he had borrowed it only two days beforehand.
Also, Lily speaks as if Sirius already knows about Dumbledore having borrowed the Cloak, yet this seems to be the first time she has written to Sirius since Harry's birthday party. So if Dumbledore really did borrrow the Cloak only a few days before the Potters' deaths, Lily would have to have written to Sirius (letting him know that Dumbledore had borrowed the Cloak), then held the birthday party, then written to Sirius again (letting him know that Dumbledore still had the Cloak), all in those last few days. Just on that basis, without regard to the evidence concerning Snape, we can probably discount the idea that Dumbledore borrowed the Cloak only days before the Potters died.
Then, the letter seems to have been written only a week or two after the Mackinnons died, since Lily refers to some bad news about the Mackinnons as a recent shock, and not just a shock to the Potters - who might be behind with the news because they are in hiding - but to Peter as well. We know the Mackinnons were killed two weeks after the Order group photograph was taken, so Lily's letter was probably written three or four weeks after the photograph was shot.
It's just about conceivable that the deaths of the Mackinnons didn't become known to the Order until months after the event, but if Lily's letter really was written just before she died, and shortly after the Mackinnons died, then the Order photograph was taken in early October. Yet, Moody lists an enormous number of things which happened between the photograph being taken and the end of the war. I go into this in more detail in a separate essay, but it's really extremely unlikely that the photograph could have been taken that late in the war. The list of events which happened between the photograph and the end of the war makes far more sense if we say that Lily's letter was written just after Harry's birthday, and the photograph was taken in early to mid July.
But that means that the Potters were already in hiding by early to mid August, and had been for long enough for James to get frustrated - and Dumbledore had in fact borrowed the Cloak some time prior to this, despite what he says at King's Cross. However you look at it, there's a continuity error. If Lily's letter was indeed written soon after Harry's birthday, then astral!Dumbledore's claim that he only borrowed the Cloak in the last week of October doesn't work, and if the letter was written at the end of October then the sequence of events which were meant to have happened between the Order photograph and the end of the war doesn't work - unless the Mckinnons' deaths, or something to do with them, were discovered long after their actual deaths - and Harry's birthday party was apparently held months after his birthday. But the idea that the letter was written in early to mid August creates fewer problems, and it fits with the fact that Rowling said at interview that Harry's christening was "very hurried", which suggests that it wasn't too many months after his birth, and that at that point the Potters were already aware that they might have to go into hiding.
It looks as though the Potters went into hiding in two stages. The initial stage happened at least some weeks prior to Harry's first birthday, and involved only a medium level of security, with friends coming and going freely. Round about the third week in October they were warned that Voldemort was about to strike, and went under the Fidelius. Despite what Dumbledore says in the King's Cross scene, he must have borrowed the Cloak no later than July 1981, and possibly earlier.
We still don't know whether Snape's defection preceded their first hiding or their second, so it still remains possible that he defected only in October 1981. But then we have to look at the evidence involving Snape himself.
Firstly, we're told that as at early autumn 1995, Trelawney has been teaching at Hogwarts for "Nearly sixteen years", and she describes herself as having been there for sixteen years both late in the autumn term of OotP and early in the spring term, suggesting that she began teaching very late in 1979 or early in 1980 As at June 1996 we're twice told that the prophecy was made sixteen years previously, so presumably in 1980. Combining these, we have Trelawney being interviewed, and making the prophecy, very early in 1980 and Trelawney starting work at the beginning of the second term a week or two later.
We're told in autumn 1995 that Snape has been teaching at Hogwarts for fourteen years, so he started round about autumn 1981. We know that Snape initially applied for a teaching job at the same time as Trelawney and that that was when he overheard the prophecy, so he must have heard it very early in 1980, while Harry was in the womb. If he didn't then defect until mid to late October 1981 that means it took nearly two years for him to realise that Voldemort associated the prophecy with Lily, and either Dumbledore hired him before he defected - in which case you have to wonder what he was doing defecting on a bare hillside instead of in an office at the school, and why Dumbledore had hired somebody he seems to have already known was a Death Eater - or he didn't start teaching until the middle of the autumn term, in which case you have to wonder what became of whoever taught Potions for the first couple of months. If he defected earlier, there's no problem.
Most strikingly, he defects on a bleak hillside on which the trees are bare and fallen leaves are blowing about. I've seen somebody say that the leaves in Virginia start falling in late August and "keep falling until November", and the otherwise generally good fanfic Snape's Boon by amr contains this to British eyes extraordinary statement: "the tree [cut] had only a smattering of yellowish leaves still fluttering in the breeze. This reassured her a little. She hadn’t lost too much time, since leaves wouldn’t hang on into November, no matter how far south she was."
This explains why American fen might think Snape defected in October, but leaf-fall in the UK happens about six weeks later (which I'm told is also true of US trees in the Deep South). A BBC nature blog on 4th October 2011 stated that "Many trees are starting to take on their beautiful autumn colour. Where just a few weeks ago there were many shades of green, we're now starting to see vibrant shades of red, gold and orange as the season shifts" but it tends to be a little later in Scotland. In fact, I've seen advertisements for people to come and look at the pretty red and orange leaves on trees in the Highlands in late October and early November - and roses blooming in Edinburgh during the first half of December.
It varies from year to year, of course, but for example a tourist page headed The Best Time to Visit Scotland for Autumn Leave says that "the leaves on some broad-leaved trees start to change color in early autumn, but the best time to see the fiery glories of fall foliage in Scotland starts at the end of September. [cut] Depending on the weather in any particular year, and the location and tree species that dominate, the specific period with the best colors can vary a lot. If you plan your trip for the middle to second half of October, though, you're bound to catch some of the best displays." The novel Death of an Avid Reader, by Frances Brady, includes the following passage set in the north of England in the first week of November: "The trees in Batswing Wood were not yet bare. Among the gold, yellow and brown, a few green leaves refused to die."
Leaf-fall has apparently got later over time. Oak trees are now recorded as shedding at the end of October, and would have done so about a week earlier in the 1970s, according to the Woodland Trust. However, the event which the Trust is recording appears to be the first significant leaf-fall when there are "some bare twigs or branches" - not the point at which the trees are bare, which would be three or four weeks later. Even in the 1970s, therefore, you wouldn't get a whole wood of bare trees in the third week of October unless there had been a very severe drought.
However you cut it, the second half of October is not a good time to find hillsides covered with naked trees and wind-blown dead leaves. Generally speaking most trees in Scotland shed during November, with a few leaves hanging on into December or even January. I caught the bus across Slamannan Moor on 12/12/2016 and I'd say about 80%, maybe 85% of the deciduous trees were bare but still there were many that still had a few leaves clinging on, and a significant minority still covered with a thick coat of brown leaves, in mid December. The rest of the UK isn't much different. This explains incidentally why Britons don't call the period from September to November "fall" - leaf-fall here does overlap with that period, but it doesn't correspond with it the way it does in the US.
[On the 31st of January 2017 on Slamannan Moor, after what was, admittedly, a mild winter, there were still hundreds of deciduous trees and bushes lining the roadside which were quite thickly covered with leaves, albeit they were brown and shrivelled. Meanwhile, this year's gorse is already flowering. Stand still for too long anywhere in the British Isles and the countryside will start growing up your leg.]
There are a few species of tree used as street ornaments in towns and which do begin to shed quite early in October - rowans and I think also hazels - but there's no reason to assume the wood is a monoculture, and assuming that that bare hill is in Britain then this strongly suggests that Snape in fact defected some time between late November 1980 and early spring 1981 (i.e. before the trees would have started to have new buds on them). The Potters then went into medium-level hiding - explaining why James was pretty fed-up by early August - until October when Snape warned Dumbledore that Voldemort was about to strike, and the Potters then went under the Fidelius.
A defection in mid to late November is to be preferred (meaning that Snape spied on Voldemort during VWI for just shy of a whole year) if you accept interview statements as at least semi-canon, because that allows for Harry to have been christened at less than four months old - just about early enough by British standards to count as "very hurried", or at least fairly hurried - and for the Potters to have already been warned at that point that they might need to go into hiding.
It is possible, if your fanfic requires it, to have Snape defect in mid October 1981 if you assume that the hillside where he defected bore only those few specific types of tree which shed early - or that it was in the US. But this assumption generates a wide range of problems which have to be explained, especially if you also assume that that was the point at which the Potters first went into hiding (the usual assumption in fanon), and that Dumbledore was telling the truth about having borrowed the Cloak a few days before the Potters died - meaning that Lily's letter, in which she says that Dumbledore "still" has the Cloak, was written at the end of October.
If Snape defected in October 1981, you have to explain away the bare trees. You have to say that either he didn't become Potions master until halfway through term, or Dumbledore hired him to teach while he was still a Death Eater. If the latter, why did he defect on a barren hillside, instead of in a nice warm office? And it would make Dumbledore's evidence in court about Snape spying at great risk to himself a bit odd, since he would only have been a spy for about ten days before Voldemort fell.
Then you have to assume either that the Potters already went into hiding long before Snape gave his warning, or that they went into hiding for the first time when they set up the Fidelius. It's possible that Dumbledore just hid any family which Voldemort might possibly think fitted the prophecy: but there's a big difference between the whole prophecy which Dumbledore heard and the fragment which Snape heard and relayed to Voldemort. Snape probably hadn't heard far enough into the prophecy (that is, not as far as "mark him") to know that "the one" was male, or even human - if he didn't get as far as "mark him" then it could just as well have referred to a weapon which was being borne, i.e. carried, to people who had defied Voldemort, and since we know for certain that he didn't hear as far as the word "equal" he certainly didn't hear the part which said that the one's birth was still in the future. It's never explained why Voldemort, based on what he had heard, ever decided that the prophecy referred to an unborn baby, let alone why Dumbledore should be able to predict that Voldemort would think it would refer to an unborn baby.
If you want to have the Potters go into hiding only after Snape gave his warning, and Snape defecting only in October, then you have to wonder why Harry's birthday party was apparently held in October, why Lily who is supposedly living under top-secret Fidelius speaks so casually of Bathilda coming round most days, why James was complaining that Dumbledore "still" had a Cloak he had borrowed only a couple of days beforehand and why Lily had apparently already told Sirius about this and was now writing to him again, referencing that past conversation, only a few days later. You have to assume that James was so immature he became frustrated about being in hiding after less than a week of it.
Internal evidence suggests that the Order photograph was taken three or four weeks before Lily's letter was written, meaning that if the letter was written at the end of October the photo' has to have been taken in early October. Given that, you have to ask how so many events could have happened between then and the end of the war, and why it would be especially noteworthy that the Mackinnons died two weeks after the photo' was taken, if most of the incidents being described happened within four weeks of it. And you have either to dismiss JKR's comment that the Potters already knew they were going to have to go into hiding at the time of Harry's christening, or assume that he wasn't christened till he was well over a year old.
On the other hand if you assume that Snape defected in late 1980 or early 1981, issuing a warning which caused the Potters to go into medium-level hiding, and their going under Fidelius in October 1981 was a separate act in response to an increased threat-level, all the rest makes sense. Snape was hired in autumn 1981, well after his defection and when he was already Dumbledore's man. He spied for around ten months, so it is reasonable for Dumbledore to speak of him being in great danger. Harry was christened when he was four or five months old. Harry's first birthday party was held on or soon after his first birthday, and Lily's letter was written in early to mid August, at which point the Potters had been in medium-level hiding for around eight months, and James had good reason to be getting impatient. The Order photograph was taken in July, leaving three or four months for all the events which occurred between then and the end of the war.
The only thing you have to work around is that King's Cross Dumbledore's statement about when he borrowed the Cloak cannot be accurate, since Lily refers to it as having happened a significant perior prior to her letter written in August. Perhaps he borrowed it twice, giving it back in between, and his old man's memory has run the two together. Or perhaps he borrowed it soon after the Potters went into medium-level hiding, and he has mis-remembered himself as having borrowed it just after they went under the Fidelius.
Snape knowingly endangered a baby when he relayed the prophecy, and other prophecy-related issues.
The full prophecy given by Sybil Trelawney about "the one" who would vanquish the Dark Lord strongly implies that "the one" is not yet born. "The one with the power to vanquish the Dark Lord approaches ... born to those who have thrice defied him, born as the seventh month dies ... and the Dark Lord will mark him as his equal, but he will have power the Dark Lord knows not ... and either must die at the hand of the other for neither can live while the other survives ... the one with the power to vanquish the Dark Lord will be born as the seventh month dies ..." Tenses in poetic speech are often a bit scrambled so it's not totally impossible that this could have turned out to have meant "will have been born" rather than "will be born at some point in the future", but it certainly creates a strong impression that it's talking about a future birth.
However, in chapter 37 of OotP Dumbledore says: "My – our – one stroke of good fortune was that the eavesdropper was detected only a short way into the prophecy and thrown from the building. [cut] // He heard only the beginning, the part foretelling the birth of a boy in July to parents who had thrice defied Voldemort. Consequently, he could not warn his master that to attack you would be to risk transferring power to you, and marking you [US version says 'again marking you'] as his equal."
This means that the very farthest Snape could have heard into the prophecy was "mark him as his..." We know Voldemort, and therefore Snape, did hear about the "seventh month dies" clause, but Dumbledore says Snape was interrupted "only a short way" into the prophecy, and "mark him" falls about 40% of the way through it, so we can probably assume Snape did not get as far as "mark him". It is likely that all he heard was "The one with the power to vanquish the Dark Lord approaches ... born to those who have thrice defied him, born as the seventh month dies ..."
Although Dumbledore says that the part of the prophecy which Snape heard was the one which foretold the birth of a boy in July to parents who had thrice defied Voldemort, because that's what it did in fact foretell, you couldn't really tell that that was what it was about if that was all you had heard.
There is no indication in what Snape heard that "the one" is male, and the only indication that they have not yet been born is that it says "as the seventh month dies" rather than "as the seventh month died". That is a minor tense-shift which might pass unnoticed or be assumed to have been misheard (by somebody who had been listening through the keyhole and was probably in the very act of being dragged away by Aberforth at that point), and it would be outweighed by the statement that the one "approaches", which sounds as if it refers to an adult champion physically travelling. Voldemort was well into his fifties by this point, he had been recruiting Death Eaters since the mid 1950s and the war had been going on for ten years, so there was plenty of scope for an adult to have parents who were young enough to be still alive and active in the war, and old enough to have already defied Voldemort three times.
It would be extremely unusual to say that an unborn baby "approaches" - indeed I think this is probably the only example of this usage that I've ever heard. Normally, you say that the birth approaches, not the baby, so to say that "the one" approaches strongly suggests an adult, or certainly somebody already born, and now travelling through distance, not time.
Snape could not even be entirely sure that "the one"'s birth was being referred to, since "born" = "given birth to" and "borne" = "carried" or "conveyed" sound exactly the same. It could refer to a champion whose parents had thrice defied the Dark Lord, or to somebody on a ship travelling to join a group of people who had defied him. It might not even refer to a person: for all Snape knows "the one" could be some powerful weapon (such as the Elder Wand) which is being borne towards them.
Or let's suppose that Snape did hear a litte further - that he heard: "The one with the power to vanquish the Dark Lord approaches ... born to those who have thrice defied him, born as the seventh month dies ... and the Dark Lord will mark him as his-". Now, OK, he knows that "the one" is human and male - but it sounds as if they're a Death Eater (therefore at least sixteen) whom Voldemort will put the Dark Mark on. Only the final "will be born" clause indicates a newborn baby and we know for certain that Snape didn't hear that.
Even Voldemort seems not to have decided that a baby was being referred to for some time after Harry's birth - or if he did he didn't tell Snape about it - since Snape defected some time during winter 1980/81, at least three and a half months after Harry was born. That fits with Rowling's original statement that the Potters went into hiding around the time of Harry's christening, since British babies are usually christened when they're a few months old. Apparently she's now saying they started hiding before Harry was born but if so it can't have been prophecy-related, since there was no way anyone (except perhaps a Seer), could know in advance that Harry would be born in late July, not early August.
Dumbledore's rant at Snape in the defection scene adds an element of confusion, especially in readers who haven't thought about the fact that Dumbledore is still so emotionally contorted about his sister's death, and anything which reminds him of it, that nearly sixteen years later he will be fatally injured after putting on a cursed ring he hopes will enable him to talk with her. In the defection scene he accuses Snape of having been willing to sacrifice James and Harry in exchange for Lily, and fanon generally accepts that statement as true - but in fact, so far as we know this is nonsense, as it was not within Snape's gift to place Harry and James in any more danger than they were already in.
The evidence indicates that the Potters were already in some sort of medium-level concealment by August 1981 and then went under Fidelius in late October, and that Snape defected in winter - which has to be winter 1980/81, since winter 1979/80 was before Harry was born and therefore before Voldemort could know that he would be born at the end of July, and winter 1981/82 was after the Potters were already dead. We've no reason to think the Potters went into hiding very soon after Harry's birth - that is to say, their decision to go into hiding almost certainly follows and is a consequence of Snape's defection, and his initial warning that Voldemort was taking an unhealthy interest in them.
Therefore, Voldemort would not need to get the Potters' address from Snape - the only way in which Snape could really have betrayed James and Harry to the Dark Lord prior to his defection - because prior to his defection they were not in hiding and their address must have been common knowledge. In fact given that Godric's Hollow is a mixed Muggle/magical community and Lily was Muggle-born and would probably want to keep in touch with her Muggle family, their address may well have been in the telephone directory.
What Dumbledore might more fairly have accused Snape of was trying to save Lily but not trying to save her husband and son. It's quite natural that a not very mature young man barely out of his teens would care more about the fate of his childhood best friend than about a bully who had tormented him for years and a baby he didn't know, but in any case he was able to come up with a good excuse for asking the Dark Lord to spare Lily - he could claim he wanted her as some sort of sexual prize - whereas it would be difficult to think up a good excuse for asking Voldemort to spare James (not one that wouldn't require Snape to follow through by killing James himself, anyway), and impossible to think of a reason why he should ask the Dark Lord to show mercy to baby Harry, the very person who had been prophecied to bring about the Dark Lord's downfall. For Snape to ask Voldemort to spare Harry would be to admit that he didn't care very much whether the Dark Lord was going to be vanquished or not, so all it would do is bring about Snape's own death and lose whatever tiny hope he had of saving Lily.
In fact, the only thing Snape could do to save James and Harry was either to convince Voldemort that the prophecy referred to someone else - sacrificing another set of innocents to spare the Potters, which really wouldn't be an improvement - or to do what he did do: to warn the Potters that they were being targeted. It's true that he seems mainly concerned with the danger to Lily but again that's natural - she's the one to whom he has an emotional connection - and he must have known, when he contacted Dumbledore, that doing so would protect (should have protected!) James and Harry as well, even if that was just a happy side-effect rather than his main goal. Nor had he known that he was putting Harry and James - or any child - in danger when he relayed the prophecy.
Dumbledore's accusation that Snape had "sacrificed" James and Harry is in fact wholly irrational and probably stemmed from his own feeling that he himself had sacrificed Ariana on the altar of his love for Gellert, and their shared political ambitions. And after ranting at and emotionally abusing Snape, Dumbledore then gave away any claim to the moral high ground by asking Snape to pay him in some way, to give him some gift in return for protecting two Order members and their child - as if the warning itself, and the risk Snape was taking to bring it, were not themselves gifts.
It is often assumed in fanon that Snape would not have cared and would not have defected had Voldemort decided to target the Longbottoms instead of the Potters. On the one hand, what canon evidence we have suggests that Snape was never a very enthusiastic Death Eater, and as a teacher he shows what is for Hogwarts an unusually strong desire to protect the students from physical danger, even when he doesn't like them. On the other hand, defection put him in danger of torture and execution not only by the Death Eaters but by the Aurors as well, so it may be that without the spur of the sudden danger to Lily he would have remained as a half-hearted Death Eater, wincing privately over the things some of his colleagues got up to and over the fact that the Dark Lord was going after a baby, but lacking the nerve and impetus to do anything about it. Especially when you see the sort of harsh, discouraging reception he got from Dumbledore when he did defect.
On what a friend of mine used to call "the third or Sellafield hand", the fact that he did choose to defect, to actually put himself into enemy hands, rather than just owl the Potters an anonymous warning, does suggest that he really wanted out, and that he had become disgusted by the organisation he found himself in and had been trying to nerve himself up to leave, even before the danger to Lily gave him the final push to do something he must have known was quite likely to result in a one-way ticket to Azkaban, even if the Aurors didn't shoot him on sight first.
The wording of the prophecy itself raises some difficult questions, and not just because nobody hearing it could really know whether the word was "born" or "borne", or because "the seventh month" could mean July, September, the seventh month of the academic year (i.e. March), the seventh month from when the prophecy was made, the seventh month of pregnancy, the seventh month of the parents' marriage and probably some others I haven't thought of yet. On the face of it, the statement "neither can live while the other survives" makes no sense, especially later on after Voldemort had used Harry's blood to reconstitute himself, and had himself become an anchor for Harry. In fact, it was more true to say that neither could die while the other survived. It only works as a self-fulfilling prophecy, since once each of them believes that they have to kill the other in order to survive, it becomes true because each is then hunting the other - even though they needn't have done.
There's a common idea in fanfiction that the prophecy was a set-up and/or that Dumbledore deliberately leaked it, and that makes a certain amount of sense. Given that Dumbledore's plan required Voldemort to attempt to kill Harry himself, not just through one of his agents, you have to wonder why Dumbledore would want to conceal from Voldemort a prophecy which would give him the idea that he needed to kill Harry himself. It's possible therefore that the original prophecy actually did say "neither can die while the other survives" and that Dumbledore concealed this from Voldemort and fed Harry a doctored version.
Laineth points out that since Trelawney saw Snape outside the door arguing with Aberforth after she had completed the prophecy, Albus must have seen him too, and yet Snape was allowed to leave without being Obliviated of the memory of the prophecy. I'm not 100% convinced of this - it may be that Dumbledore tried to Obliviate him and failed because Snape was already an Occlumens - and nor I think do we know whether or not Dumbledore already knew that Snape was a Death Eater. But if he did know, and did allow Snape to leave without being Obliviated, then it was Dumbledore who allowed Voldemort to know about a prophecy that an as-yet-unborn baby would be a threat to him.
Of course, since Snape didn't know the prophecy referred to a baby, Dumbledore might not expect Voldemort to work that out from the little that Snape had heard - but did Dumbledore know at that point that Snape had only heard half of the prophecy?
Snape would have known that Pettigrew was the spy in the Order.
This one was brought to my attention by an essay on Half Blood Ponce's blog. Many fen assume that Snape must have known that Peter Pettigrew was Voldemort's spy in the Order, knew that Sirius was an innocent man in Azkaban and could have saved Lily and James if he had only told somebody. But in the trial scene which Harry views in the Pensieve in GoF, Karkaroff says "You must understand that He Who Must Not Be Named operated always in the greatest secrecy ... he preferred that we – I mean to say, his supporters [cut] – we never knew the names of every one of our fellows – he alone knew exactly who we all were –" and Moody comments that this was "... a wise move, wasn't it, as it prevented someone like you, Karkaroff, turning all of them in."
It's unlikely Karkaroff is lying, as he is trying to shop as many people as he can in order to curry favour, not making up excuses not to shop people. It's not impossible that Snape knew about Pettigrew, but if he did it raises all kinds of plot issues, and it's clear that it's wholly possible he knew nothing about it. If you take authorial intention into consideration, it seems likely that Rowling introduced this piece of conversation in order to show that the reason Snape didn't tell anybody about Pettigrew was because he didn't know.
Indeed, if Voldemort knew about the dynamic between them, the fact that Pettigrew had been part of the gang that bullied Snape, he would surely make sure Snape didn't know what Pettigrew was doing in case he was tempted to betray him out of spite.
The situation is complicated by the possibility that Snape himself might have been responsible for some of the Order leaks during Vold War One. It's strongly implied in Vold War Two that Dumbledore has Snape betray some genuine Order secrets to Voldemort in order to win his trust.
