[spoilers] I think Snape is indeed faking being on the 'dark side'. I think he's still working for Dumbledore.
Dumbledore trusted Snape
But I think he didn't give Harry the exact reason why. He said it was because Snape realized he was responsible for Voldemort going after James and Lilly. But taking into account that Snape hated James, I think it must be about Lilly. I think Snape was in love with Lilly. This would make Snape turn sides when he realized what he had done. And since Dumbledore values love over all, this would be reason for him to trust Snape.
Snape makes the Unbreakable Vow
Snape knew he had to kill Dumbledore by making the Unbreakable Vow to Narcissa. Yet he hesitated. If he was totally evil, would he have hesitated? I don't think so. I even think Dumbledore sent him.
Dumbledore pleading to Snape
Dumbledore pleaded to Snape. I cannot see Dumbledore pleading for his life, since he doesn't fear death. Could Dumbledore have ordered Snape to kill him to save Malfoy. I think perhaps Malfoy is to play some important role in book 7, so they cannot have him die, or kill Dumbledore.
The argument Hagrid overheard
Another clue to the pact between Dumbledore and Snape. Snape said he doesn't want to go through with it. I think this is about killing Dumbledore.
The look on Snape's face when he had to do it
A look of revulsion and hatred. I think the revulsion and hatred are for himself. Not for Dumbledore.
Snape being hurt when Harry calls him a coward
Snape chose to do what is hard, not what is easy. And still Harry calls him a coward. That must hurt.
Doing what is best, not what is easy
A recurring theme in Harry Potter. I think this is what Snape did. I think he would have preferered death over killing Dumbledore. Try to imagine Snape's life. Being responsible for the death of the two people you have ever loved... [/spoilers]
Personally, I don't believe that Snape is actually evil. I think that he and Dumbledore both knew exactly what had to happen. I agree with Aidoann that his reason for not going evil had something to do with Lilly. I've got a bunch of theories that I'm too lazy to flesh out right now, but maybe I will later.
^I Made This.^
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