Meta: 'But Snape is just nasty, right?'
As a grown man: He died pointlessly
We know Snape spent his dying breaths (assuming he really was dead and not just unconscious) still doing his duty and giving Harry the information he needed to complete his task - and also to show him the truth of his friendship with Lily. It's ambiguous whether that final "Look at me" was a plea for Harry to let him die gazing at Lily's eyes, or for Harry (or Lily-in-Harry) finally to see him as he truly was.
The fact that the memories he gave to Harry included the information that he had been trying to protect Remus when he cut George's ear does suggest that he cared about what Harry (or perhaps Remus) thought of him, and wanted somebody to understand that he wasn't at all the cold-hearted bastard they often thought him. It wasn't just to explain why he had injured an Order member, in order to get Harry to trust him. The memory of portrait!Albus telling him that if he became embroiled in the chase he would have to make it look convincing would do that, and could easily be verified by asking Albus. Nothing was changed by the revelation that Snape had disobeyed Dumbledore and risked his cover to save Remus - except his reputation in Harry's eyes and those of his former colleagues.
Similarly, he didn't really need to give Harry those memories of Lily, from a practical point of view. He needs to make Harry trust him but it would probably have been enough to show Harry Dumbledore ordering his own death, and discussing the Horcrux in Harry and why Harry too had to die. The information that he was friends with Lily - and in such detail, showing their childhood conversations and quarrels - has little practical benefit, but it makes a memorial for those children and that friendship, it lays bare both Snape's love and what he feels to be his fault, and makes Harry a final gift of his mother's memory, echoing the photographic book of memories which Hagrid prepares for Harry in the first book.
Was his death itself sacrificial? Did he have a choice about whether he died or not, or only about how he conducted himself in death?
When his colleagues attack him and drive him out, he responds to their attacks effectively and with lightning swiftness, but all his moves are defensive ones and he does nothing to harm them even when they are throwing knives at him. Presumably, then, his reason for fleeing is because he doesn't want to hurt them, not because he is unable to fight back, so he is already making a sacrifice: exiling himself and sending himself out to stand among the Death Eaters who are in fact his enemies, rather than hurt the people who used to be his friends.
He can't come right out and tell them he's on their side, of course, even if they would believe him - which they almost certainly wouldn't. If Voldemort won, Snape would have to stay on at Hogwarts to protect the children, so he can't reveal his true allegiance to people whose minds are vulnerable to Legilimency.
When we see Snape with Voldemort in the Shack, he is still trying to do his duty and get to Harry to give him Dumbledore's message. Initially he seems tense but calm, but we see him become progressively more nervous as the conversation continues. He begins to stutter slightly when Tom says that the Wand is not performing any remarkable acts for him, and he turns deathly-white when Tom begins to talk about it by name as the Elder Wand. JKR seems to be deliberately showing us that the mention of the Elder Wand, or the demonstration that Voldemort knew what it was, had special significance to him and frightened him especially. Did Snape then understand the situation with the Wand?
Dumbledore tends not to tell anybody anything if he can help it. Nevertheless, it seems clear that he wanted Snape to learn about Horcruxes, or to be reminded about them. Whether as Dark wizard or DADA teacher Snape certainly has the reputation of being academically interested in Dark Magic, and Dumbledore left a stack of Dark Magic grimoires with sections on Horcruxes in them in his office, which he knew would soon be Snape's office. He couldn't have known in advance that Hermione would steal them, and he must have known that if they were still there when Snape took over Snape would read them - so he must have wanted him to.
Why did Dumbledore want Snape to think about Horcruxes, and yet wouldn't tell him about them himself? He said himself that he was wary of entrusting secrets to somebody who spent so much time so close to Voldemort. Presumably, therefore, he didn't want to tell Snape about the multiple Horcruxes himself because he didn't want the risk of Voldemort breaking through into Snape's true mind and finding out that he, Dumbledore, knew about the multiple Horcruxes. That would be dangerous for the cause and absolutely fatal for Snape, since there could be no reason for his failure to pass on the fact that Dumbledore knew such an important and (to Tom) threatening thing, other than his true loyalty to Dumbledore. On the other hand, if Tom saw in Snape's mind that Snape had read a book about Horcruxes and had speculated about them in relation to Tom, with no apparent reason to think Dumbledore had reached the same conclusion, there would be nothing suspicious about his failure to raise the matter.
To some extent it would be dangerous for Snape if Tom found out that Dumbledore had told him that Tom couldn't die while Harry lived, and Snape hadn't passed it on - but he could at least say "But master, I knew, because you told us so, that you had taken other steps to make yourself immortal and that you would still live even if Potter died." In any case, he may have passed that information on - it would explain why Tom had ordered the Death Eaters to leave Harry for him. But Tom, too, would think that he was safe to kill Harry, because he had other Horcruxes and to the best of his knowledge, Dumbledore didn't know about them. Even in the final battle, he probably still thought the diadem was safe.
So, we can see that Dumbledore apparently did want to keep Snape informed, where it was useful to the anti-Voldemort cause for him to be informed, and where his being informed would not unduly endanger either the cause or Snape himself.
Although Harry tells Voldemort that Dumbledore expected the mastery of the Elder Wand to die with him, what he says to Dumbledore is that Dumbledore expected the wand to "end up" with Snape, and Dumbledore agrees with that. It does not sound as if they are talking about mere physical possession of a stick - especially as they are discussing it in the context of Dumbledore and Snape making mutual plans - and at any rate Dumbledore couldn't know in advance whether ordering Severus to kill him would cause the mastery to die with him, or to go to Severus.
If there was a good chance of Snape ending up with the mastery of the Wand, then you would think that he would be much better equipped to make informed choices about it, and therefore to be as useful to the cause as he potentially could be, if he actually knew about it. Since there is evidence that Dumbledore did want to keep Snape informed, where it was both useful and safe to do so, he would probably want to tell him about the Wand - if it was safe to do so.
Was it safe? Well, so far as we see Tom Riddle did not begin to take an interest in the Elder Wand until after Dumbledore's death, so they had no reason to think that the Dark Lord would specifically probe Snape for information about Dumbeldore's wand (as it were). In the other hand, if Dumbledore told Snape about the Wand weeks or months before his death and Voldemort for some reason chose to interrogate him, there was a risk he would find out Snape was keeping this vital secret from him.
However, although Dumbledore could not have known the exact day of his death, he did know he would die by the end of term. So long as he told Snape towards the end of term the risk of Snape being caught concealing information by Tom in the short time remaining would be low. Once Dumbledore was dead, if Draco had not intervened the mastery of the Wand would either have died with Dumbledore or passed to Snape. If it died with Dumbledore, Snape had the perfect excuse for not having told his master about a weapon which no longer existed anyway. If it passed to Snape, the danger from Tom would be much less anyway, because Snape now had an unbeatable weapon. So there was no very strong reason not to tell him.
Of course, even if Albus did intend to tell him about the wand he may have died before he had the chance to do so - but then portrtait!Albus could have told him about it later. By that point, however, he would know that the mastery had gone to Draco, so telling Snape about it would put him in at least some danger, since he was concealing the existence of a dangerous weapon from Tom, without himself being protected by it.
Even if Albus deemed it too risky to tell Snape about the Wand while Tom did not know about it, once it became apparent that Tom had broken into his tomb - which happened about a month before Snape's death - then it would be both advisable and safe for portrait!Albus to explain what was going on, so Snape would be forwarned. And even if neither he nor portrait!Albus knew that the Wand had been retrieved and buried with Albus, once the tomb was broken into they would have reason to find out, and to surmise that that was what Tom had been after.
Once Snape had reason to think Tom already knew that Dumbledore's wand was the Deathstick, he couldn’t be accused of keeping the identity of the Wand a secret. In which case, it was safe for him and Dumbledore to discuss the disposition of the mastery. It shouldn’t matter that Dumbledore had asked him to kill him, or whether or not he had told Tom about that, or that Dumbledore had thought the mastery of the Wand would go to him, because Voldemort knows that Dumbledore thinks Snape is Dumbledore’s man. All Snape has to say is "The old fool thought that I was killing him on his own orders but really I killed him for you my lord: I was merely a weapon in your hand, and the mastery should have come to you." Depending on how intelligent and responsive the Wand is, if Snape truly had the mastery then it might actually work for Tom well enough to fool him, if Snape wished it to do so and doing so served Snape's interests.
Knowing the mastery had probably passed to Draco, and not telling Tom about it, would still be risky, but Snape could at least say "I did not tell you because if I exposed him to your wrath I would have broken my Vow to protect him, and I would die, and besides my lord I thought you knew that Draco had disarmed him." Tom would not be happy about that, but at least it wouldn't make Snape a traitor and it wouldn't be as bad as not having told him the Wand even existed, so he might get away with it. The risk, at any rate, would probably be less than the risk which would arise from not knowing what was going on, once Tom actually had his hands on a weapon that wasn't going to work for him.
From the way Snape is described as being especially white and scared when Voldemort talks about the Deathstick, it does look as if JKR means us to understand that he knows what Tom is talking about and knows that this topic is dangerous to him personally: and I have shown that it is perfectly possible, even quite likely that he does know. If so he had at least a month to think through all the ramifications, and may have had a year. And in that case, Snape could have saved himself, or at least tried to - but he chose not to.
If Snape knows anything about the mastery of the Wand, he knows that it didn't go to him - unless he thinks Dumbledore had simply dropped it, but he's probably heard what really happened from Draco, or from Dumbledore's portrait. So he knows the mastery went initially to Draco, although he may or may not know what happened to it next.
He can't say "Draco has the mastery of the Wand", because then Tom would seek out Draco with a view to killing him - and Snape has taken an Unbreakable Vow to protect Draco as best he can. But it's clear Tom doesn't know the details of what happened on the Astronomy Tower, so Snape could perfectly well say that Dumbledore was disarmed by the Death Eater who was later killed in the fight, and that the mastery went to whoever killed him. It might or might not work, but it would have at least a chance of working long enough to buy him time, and one way or another Snape has had a year to think up a good excuse - so why doesn't he even attempt it?
Snape may or may not know that the mastery has gone to Harry, but he knows (if he knows anything about it at all) that he doesn't have it, and that if Voldemort thinks he can get the mastery by killing him, Voldemort is wrong. Presumably, then, if he knows what's going on - as his whitening at that particular point suggests - he chooses to keep silent and let Voldemort kill him, rather than warn Lily's murderer that he is about to go into battle with a weapon which will not answer to him. This would explain why Rowling said that Snape laid down his life because of love - which sounds likes something a lot more intentional and controlled than simply gambling on whether or not Voldemort would kill him, and losing.
As a grown man: He killed Dumbledore
We know now, of course, that Snape was definitely following Dumbledore's orders when he killed him; and we can also be virtually certain that Dumbledore had already given him this order before he took the Unbreakable Vow.
Both events - Dumbledore's injury, followed within the hour by his decision that Snape would have to kill him, and the taking of the Unbreakable Vow - occurred between the fight at the Ministry of Magic in mid June 1996 and Dumbledore's collecting Harry from the Dursleys in mid July 1996, by which point Dumbledore already had a withered hand. The Spinner's End chapter takes place in July (Snape refers to the Dumbledore/Voldemort duel at the climax of the Ministry battle as having occurred "last month"), some time prior to Dumbledore collecting Harry.
In the Spinner's End scene, Snape refers to a serious injury which Dumbledore has sustained since the Ministry of Magic fight. Since it is most unlikely that Dumbledore would have sustained two serious injuries in one month, one of which we aren't told about, we must assume Snape is talking about the curse-injury to Dumbledore's hand, and that this is after Dumbledore arranged his own death with Snape.
Dumbledore certainly already knows, on the night he puts on the Peverell ring and sustains the curse injury, that Draco has been ordered to kill him, and he discusses it with Snape, who presumably told him about it in the first place. So Snape knew what he was swearing to when he took the Vow, and he also knew that it was what Dumbledore wanted. His flinching jerk of the hand when Narcissa asks him to carry out Draco's task if Draco should fail, then, must spring from reluctance to obey Dumbledore's order - or perhaps he has been thinking of refusing to kill the old man, and realises that now if he does refuse, he himself will die.
We do not know whether Snape ever tells Dumbledore about the Vow. When Harry tells Dumbledore that Snape has taken an Unbreakable Vow to protect Draco, Dumbledore implies that he already knows, and there is no nefarious reason why Snape would fail to tell him. It's not as if he's biding his time, preparing to sneak up on Dumbledore unexpectedly in order to save himself: Dumbledore wants him to kill him. And yet, when they argue about it in the Forest, Dumbledore reminds Snape of his promise to him, of "services you owe me": he does not say "You know that unless either yourself or Draco kills me, your own life will be forfeit".
This suggests one of two things. One is that Snape has not told Dumbledore about the bit of the Vow where he swore to carry out Draco's mission if Draco failed. Under the circumstances there seems no good reason why he wouldn't tell him, unless it is that he is still hoping to get out of killing the old man, at the probable cost of his own life, and doesn't want to hand Dumbledore another arguing-point.
The other is that Dumbledore does know about it, but he doesn't use it as an argument because he knows Snape regards his own life as of little importance. Either way, we know about the Vow, and we know Snape is trying to get out of killing Dumbledore - so we know he is willing to die for the old man, or in order not to be a killer.
At the Astronomy Tower we see Dumbledore actually plead with Snape. It could be that he is faking it in order to make his supposed murder look more convincing, but if he really is pleading then he fears that Snape will still refuse to kill him at the last moment. If he knows about the third clause of Snape's Vow, then he thinks that Snape will choose to die to buy him a few more weeks of life - and he certainly believes Snape is very reluctant to kill.
We also see that Snape is concerned that killing might damage his soul, so he certainly isn't a killer now. Probably he never was - or if he ever was, he desperately doesn't want to repeat it. He is not a particularly ruthless man now, and probably he never was.
It was necessary for Snape to kill Dumbledore, or at least they thought it was: and not just for humanitarian reasons. Dumbledore believed that the curse which was on the Peverell ring was placed there by Tom when he put his Horcrux into it ("I quite forgot that it was now a Horcrux, that the ring was sure to carry a curse"), so if he allowed himself to die of that curse, it would be Tom who had killed him, and so the mastery of the Elder Wand would go to Tom, which he desperately wanted to avoid. Even if he committed suicide, he would be doing it because of the curse, and so there would be a risk that it would still count as Tom having defeated him.
Getting Harry to feed him poison may have been insurance: if Snape refused at the last minute to kill him, then his blood would be on Harry's hands and either Harry would get the mastery, or it would die with him because he had ordered his own death. He couldn't know for certain, in advance, how the Wand would interpret a death he himself had ordered, so he arranged matters so that if the mastery didn't die with him, it would go to Snape, or failing him, to Harry.
In the event, of course, it went to Draco, so Snape's killing Dumbledore didn't make a difference to the disposition of the wand: but there wasn't time to rethink their plans, and by that point, in any case, Albus really was in a position where he might be killed slowly and horribly, and really needed the mercy he had asked Severus for.
To anyone who still hates Snape for killing Albus, even though it was on Albus's orders, consider this. An hour or so beforehand, on the man's own orders, Harry force-fed Albus the potion from the Eldritch Birdbath of Doom. This certainly contributed to Albus's death, by making him too weak to defend himself.
If Snape had not killed Dumbledore, and Albus had died some hours later from the poison which Harry made him drink, would you transfer your hatred from Snape to Harry? If not, why not?
As to why Snape stared at the Headmaster in apparent hatred and revulsion - we know he was not a natural killer, so he would have hated what he was having to do. He also had much to resent the old man for, for we see that Albus was sometimes very cruel to him - and also he'd be angry with Albus for emotionally-blackmailing him into doing something he didn't want to do. This is Snape we're talking about, after all - and nobody ever claimed he had an easy temperament.
But if the fact that he is capable of hating Albus is a moral fault, then the same must apply to Harry - for here, too, they mirror each other. At the end of OotP, Harry too feels violent hatred towards Albus for his implacable calm when Harry has just lost a beloved father-figure, but Snape has been commanded to be the author of his own bereavement.
IRREDUCIBLE INDICATIONS OF VIRTUE
It is no longer necessary, as it was in the earlier version of this essay, to provide evidence of Snape's true allegiance to the anti-Voldemort cause. We know now that he was, very literally, faithful unto death. To what extent, though, were his good actions driven solely by his love of and guilt over Lily, and to what extent was he moved by wider considerations? Was he (despite his temper, his sourness etc.) a good and intrinsically loving person - or just an obsessive one?
Snape is not an emotionless person, nor is he lacking in fear. When he defects to Dumbledore we see him utterly terrified by his own situation and Lily's. He turns white when Harry nearly knocks him off his broom, and again when he has to return to Voldemort, and he is sheet-white and stammering as he confronts his own death. Yet he still returns to danger, again and again, to do what he thinks is right.
Snape hates Sirius, and he has every reason to - yet when Sirius is unconscious, Snape summons a stretcher for him. He believes that Sirius is a mass-murdering Death Eater who is responsible for Lily's death, and he sincerely wants him worse than dead in return - yet when it comes to it he, unlike Sirius, cannot bring himself to be rough with an unconscious prisoner.
Later, even when he knows Sirius wasn't a traitor, it remains true that Sirius is a bully and a would-be murderer who tormented him as a boy, who tried to kill him and who remains unrepentant about it. Yet when Snape finds out that Umbridge is investigating Sirius, Snape takes the trouble to warn Dumbledore (as we know he did, because Dumbledore knows about it at a point at which his information could only have come from Snape).
Snape's position under Umbridge is a precarious one. He could easily lose his job - and if he did, he would also lose most of his usefulness to Voldemort, especially if Dumbledore were not reinstated. Yet Snape incurs her wrath by messing her around over supplying Veritaserum. That to some extent protects Harry as well as Sirius. But there is no benefit to Lily's son, or to the cause, when Snape, already on probation, risks angering Umbridge further by intervening to prevent Crabbe from throttling Neville.
He has a sincere grievance against Remus, who took part in persecuting him as a boy, and undermined him as an adult. He had no qualms about getting Remus sacked once it was evident that the man really was a danger to students. Yet when he thought Remus's life was in immediate danger, he risked blowing his cover in order to protect him - even though doing so ran contrary to Lily's interests, and those of the Order. As with Sirius, when he sees someone in danger his gut instinct is protective, even if it's an enemy. And he cares that people should know that - or perhaps that George should know he didn't injure him deliberately - so much that he includes it in his dying testimony to Harry.
He likewise risks drawing the Carrows' suspicion down on himself by giving Neville, Ginny and Luna a detention which is more of a reward, after they try to steal Godric's sword. His motives are, perhaps, always personal - he is moved by concern for individuals, rather than by any wider political picture. But he shows concern for all individuals, not just for Lily and her son, and he fights Voldemort because Voldemort is something from whom individuals need to be protected.
We see this when Dumbledore reminds him of his promise to stay and protect the school - even though that is not the course of action which best serves Harry. We see it when he says that since he ceased to be a true Death Eater, he has only watched people die if he could not save them - that is, he saves the Death Eaters' victims where he can, even though doing so does not serve Lily's interests, and risks blowing his cover.
We also see it when he runs through the castle in his nightshirt because he heard somebody scream, even though he didn't know (until he got there) that Harry was in any way involved; and when he sprinted through a closed bathroom door, not knowing who or what might be on the other side and probably again not knowing that Harry was involved, or anything except that a girl's voice had screamed "Murder!"
We know, also, that his dedication to Dumbledore and the Order is sincere, even if it centres around his loyalty to Lily, and even if Dumbledore to some extent emotionally manipulates him into it. It cannot be the case, as some people have suggested, that he has to obey Dumbledore because he is in the old man's power: from the point at which Dumbledore let Snape know that he had connived at Sirius's escape, if not before, Snape had far more serious blackmail material on Dumbledore than vice versa. We have plenty of evidence that he had never been a very enthusiastic Death Eater, and the Ministry had already pardoned his past and rubber-stamped it, so there are no dark secrets in his past that Dumbledore didn't order him to: but he knew all manner of interesting things about Dumbledore and the Order which the Ministry would have loved to know. The fact that Dumbledore was nevertheless so offhand with him shows how deeply he trusted him - and his trust was not misplaced.
It isn't simply that Snape will protect the people under his care from danger of death: he will protect them from pain if he can, despite his own sometimes spiteful nature. It is noteworthy that the lever which Albus uses to persuade Snape to kill him is not political strategy but an appeal to his innate mercy - that Snape should give him the quick, painless death which the man himself can hardly hope for.
In the scene where Harry pursued Snape and the Death Eaters across the grounds after Dumbledore's death, the reason which Snape gave for stopping one of the Death Eaters from Cruciating Harry was clearly bogus. It might be true that Voldemort wanted to deal with Harry himself, but there was no reason why the Death Eaters shouldn't have their nasty fun, without killing him. But Snape took the trouble and the risk of protecting Harry from suffering, not just from danger - even though he himself ended up giving Harry the magical equivalent of a slap for calling him a coward (which is rather less violent than the hex which Remus throws at Harry in DH for calling him the same, btw).
And then, as Snape fled, Buckbeak tore at him, and Snape did nothing to defend himself. Even if he hadn't the energy for another Avada Kedavra you would think he would be able to do something to drop a flying predator the size of a horse which was trying to maul him, but he does nothing. Just as he would not fire on Minerva and Filius even when they were trying to kill him, so he was apparently willing to let himself be mauled, rather than either harm an innocent animal or upset Hagrid by hurting his pet.
And then, in the end, Snape does as Dumbledore tells him, and gives Harry the information which he thinks will send the boy to his death. We know he does so most unwillingly - we see his shock and anger in the Pensieve - so he doesn't act out of carelessness for Harry's safety, whether he likes the boy or not. It must be because he has been persuaded that Dumbledore is right. He began by pledging anything and, by implication, everything to Dumbledore and the Order for Lily's sake, and ended by giving up even his dedication to Lily's memory, for Dumbledore's and the Order's sakes.
So here we have our man - stressed-out, ill-tempered, sarcastic and harsh, sometimes childish and often spiteful but also brave, protective, ethical, dutiful and self-sacrificing. He's certainly no angel - but his virtues compare well with the best that the light side has to offer, and his faults are arguably less than those of many other "good" characters. Being a bit petty and spiteful hardly compares with trying to murder someone for kicks, or covering up information which could save a child's life, just to save face.
Ultimately, the story pivots on him. Without Snape's loving devotion to a friend who had rejected him, his willingness to risk everything to plead with the Dark Lord for her life, Tom Riddle would never have offered Lily the choice of whether to live or die, and there would have been no Blood Protection, no sudden disembodiment of Voldemort, no Harry. Of course, if Snape had never relayed the prophecy in the first place the Potters wouldn't have been singled out, although they would still have been in danger as Order members: but whether Tom had pursued them and killed them all, or ignored them altogether, without Snape's intervention, and Lily's willing sacrifice which flowed from it, the Death Eaters would have swept to victory in the 1980s, and wizarding Britain would have been ground under the heel of an immortal dictator.
Without Snape's devotion and courage, his willingness to spend his last few seconds of life passing on the instructions which would enable Harry to win the war, Harry might still have lost the final battle. "The Flaw in the Plan" which brought Voldemort down, and which was the final chapter-title and the culmination of the series, was Snape's capacity for love and sacrifice, and Voldemort's failure to anticipate it. Even Harry's signature Expelliarmus, which carried him to victory, is a spell he learned from Snape.
I once characterized Snape to the writer duj as "Doing the Right Thing with exemplary courage whilst bitching about it the whole way" - to which she replied "That's our Snape, all right".
