This is my theory/analysis of Snape as of the end of book 5 (June 2003)
He’s a Slytherin kid at heart, perhaps with the same upbringing (hating mudbloods, etc.) but is also very intelligent and gifted in other ways. He doesn’t seem to have any friends (“very unpopular” was the term Harry used in the memory), choosing to isolate himself when he has the chance, even hide at times. He seems to have the strength of will, but perhaps not the speed or ability to fight back. Remember the small boy crouching as his father yelled at his mother; perhaps he was physically beaten or abused as a child as well. Either way, it doesn’t seem hard to get from a crying child in a corner to having the nickname “Snivellus”. Perhaps his father followed Voldemort, even. In fact, it seems likely that Snape would want to take after his father who was clearly in control of the family. But as a child, he is at a loss for power- and power is something that Slytherins cherish. Power and strength and control- huge deals to anyone born with a Slytherin-type upbringing and things that would allow Snape to break free of the ridicule and bullying and difficulties he experiences at home and at school.
But it’s not until he joins Voldemort that he’s offered a way to be in control and have that power. It’s his “only way out” towards what he really wants. He’s been bullied (by no less than the great James Potter, Quidditch Cup Winner) and I think he can understand a lot of the Marauders’ weaknesses because of that. Remember the “You wait! You wait!” threats as James bullied him in the memory. And also remember how he doesn’t want Lily to stand up for him- he would rather turn evil and get even than have a girl (and a mudblood no less) pity him and help him. When he tries to retaliate- he doesn’t just jinx, he makes James bleed- an obviously violent spell. A dark art, if you will. Anyway, he joins Voldemort, gets power and control and is able to do things he could have only dreamed of before. The whole bit.
However, after a while, he realizes he’s just a big a bully as James… the power didn’t make him as happy as he thought. The control is all a negative sort of control. And if he had any thoughts to use the power and control differently, he finds he cannot (or is not supposed to). Then realizes he’s just as controlled as anyone else, controlled by Voldemort. So Snape’s is not true power, and he’s lost freedom of will in the meantime, again. It’s something like this that makes him want to break free. Perhaps it’s a certain incident, or something more specific. But somewhere along the line, he learned both Legilimency (which he obviously knows because he uses it on Harry) and Occlumency (which Lupin says he is a master of). More importantly, it is the Occlumency which allows Snape to break away from the Death Eaters and “turn spy” without them sensing or knowing. As Voldy is big at Legilimency and can thus detect when people are lying to him, we can only assume that it is Snape’s abilities to counter that with Occlumency which allow him to be a spy at all and not be killed on the spot for it.
As for why Dumbledore trusts him, I have always believed it was some huge event that transpired. But it may simply be that Snape understands that he is being used, and decided on his own to block that control. He decided he wants to break free, and is thus in a position Dumbledore can take advantage of him in. Dumbledore would trust him because it was Snape’s decision to break free in the first place. This does not mean that he is all good… just that he is good enough and trust worthy to some degree. But for Dumbledore to show as much care/concern for him as he does in the books, and to trust him with Harry’s life, I am convinced that some large event of some sort transpired to bring about this realization and change in him.
Which means he could very easily be a “good guy” completely now. But at the same time, he could be a double agent easily. Now the question is, if he did decide to break away from Voldy for internal rather than external reasons, why would he want to go back to that side? Voldy can offer him nothing but to have a master. Really, the prospect of torturing and killing Harry is all that Voldy could possibly offer, and Snape can technically do that now, having been alone with Harry loads of times.
Now my question is, how does he still get away with working for Voldy without being suspected? All right, he can block Voldy from knowing he’s a spy. But how does he explain working for Dumbledore now at Hogwarts? And how does he explain not seeking Voldemort out, as a faithful death eater? Voldy goes through a whole long thing with Barty Crouch Jr. to get him inside Hogwarts. However if Snape is still evil (or Voldy thinks he is), he’s already got a “faithful” death eater inside Hogwarts. And with such things as Snape working against Voldemort in the first book to Quirrel(and thus, Voldy)’s face… how in the world would Voldemort believe that Snape was still on his side? He must know that Snape is working for the good guys. And assuming he knows, there seem to be only two options. One, that he is allowing Snape to put on a farce and pretend to be back on his side, when he knows he’s just spying for Dumbledore. Voldy could just feed him misinformation here and there at times, if that is the case. Or two, that Snape is really a double agent and is indeed working for Voldemort. Then why wasn’t he the one used as the insider in book 4? Perhaps because Voldy didn’t want to give away such a valuable source of inside info. Perhaps Snape was so perfectly set up to be a double agent that he could not be so active yet.
You have to wonder why Snape did not flee like Karkarov in book 4 when the Dark Lord returns. Obviously, people who know him and Dumbledore believe he is good, a spy, and thus not a threat. Perhaps he is so filled with the desire for revenge against Voldy and to do the work of good that there’s no need to run. Yet he does not go to the Dark Lord’s side then (he can’t apparate off school grounds and can’t immediately leave or it will be suspicious) but you must wonder if the reason is because he does not fear Voldy’s return, because he’s still working for him. And then, you must wonder what Voldemort says to him when he does finally return.
I dunno. I still can’t decide if he’s good and his loyalties are truly and absolutely with Dumbledore. Or if he’s bad and is just using his position to get to Harry and help Voldemort (like with the Occulmency). He’s supposed to be teaching Harry… but doesn’t really do much teaching. And I honestly think that it is not because he is a bad teacher or Harry a bad student. This may reflect the concept that he learned it on his own, so he thinks Harry should be able to learn it the same way. But the lessons don’t seem to help at all, and make things worse on the nights when Harry tires it. It’s certainly possible that Snape was using the lessons to help Voldy gain more control- to get Harry to a vulnerable point, then stop the lessons and allow Harry to be lured. Snape is at Hogwarts with Harry… yet doesn’t manage to stop Harry and the others from leaving Hogwarts to “rescue Sirius” either. The other possibility is that Dumbledore wants Harry used, to lure Voldemort out, and Snape is using Harry in that way. Or perhaps Dumbledore is evil in his own way and is ordering Snape to make Harry vulnerable because he wants the evil to occur.
I really can’t say. But I think it’s also very possible that Snape is faithful to the order. It’s slightly less (based on my reasoning about why he left) possible for him to go back to the evil side. However, this all hinges on my idea of why he left, which could be completely wrong. Any number of specific (‘You killed my father, prepare to die’) or general incidents (‘I’m getting nowhere with this, maybe I’ll try Dumbledore’s side for a while’) could account for him leaving, and thus he really could be good or bad possibly just as easily.
If you look at the details… Snape came to Hogwarts school to teach, under Dumbledore’s invitation, in the year of Dumbledore’s downfall. That’s at most 3 years after his graduation- that’s extremely young to become a teacher in one’s old school. Dumbledore trusts him with the kids, and with potions… but not the Dark Arts. He thought that then, and now (and every year in between), that it might have the result of forcing Snape onto the side of evil. As a kid he was “up to his eyeballs in the dark arts” as Sirius puts it. This supports the theory that then, and now, Snape does have the desire to control and be in power. And when we come down to it, he is a Slytherin, still. Gryffindors will fight to save others… Slytherins will put themselves first and save themselves above others. (Or so Phinneas the painting told Harry). Why then would Snape be working for the Order when he could be safe and getting ahead with Voldemort? There has to be something… perhaps it is only that he wants to be in complete control of himself… or perhaps it is something different. But if it’s complete control over himself that he wants, he’s sure not getting that taking orders from Dumbledore, either.
Either way, Snape could be good or he could be bad. I still can’t completely decide which. But I like my theory of why he’s where he is now… even if I don’t know exactly what he’s doing right now. If I had to say one way or the other, given the evidence, I’d have to go with that he is truly working for the side of good. In which case, I have no idea what Lord Voldemort thinks about him for taking this job at Hogwarts. A lot of it hinges on trusting that there is some incident that explains the turn and the trust. And a lot of it hinges on knowing is Dumbledore is good or not. They could both be good or they could both be bad. Snape could be bad and Dumbledore just isn’t seeing it, or is using him as a double agent unknowingly and feeding him misinformation. Or Snape couple be good and Dumbledore could be bad and is using Snape, easy to do considering his weaknesses. It’s all just very difficult to say. I just wish I knew more.