I have a wacky theory about Snape, regardless of his past and the way he acts now, I dont think he is evil at all, I think he is just a miserable git, during the first two books I noticed a pattern where although it seemed Snape was making Harrys life harder it actually dawned on me that all he ever does is push Harry to his limits, as If he is trying to prepare him for something. You remember when he whispered in Dracos ear and made him summon the snake, i feel he did that to find out if Harry could really talk parseltongue, of course making Harry think he still has it in for him.
I think that Snape has resentment for Harry, not because of the past events with his father, but because he didnt get sorted into Slytherin where he could keep an eye on him. Harry always seems to be running into snape all the time, even when hes sneaking around under the cloak, Snape is watching out for Harry but not because he wants him to come to any harm. I really think there is going to be some huge plot twist in the final book thats going to reveal a few characters for who they really are.
Did anyone else notice how Dumbledore almost always seems to end up leading Harry right to Voldemort? I dont think Dumbledore or Snape are really who we think they are.