Well, I keep threatening to write my own version of Harry Potter 7, because I just can't wait until Ms. Rowling gets around to writing it. I have lots of interesting ideas, some of which are original, and some of which, I admit, I got from reading every word everyone has ever written about Harry Potter. If you haven't read all of the books released, until now, reading what follows could ruin it for you. And no, seeing the movies is not even remotely adequate.
For many people, this is probably a silly question. After what Snape pulled in the closing pages of Half Blood Prince, many people finally believe what Harry has been thinking all along. There are, of course, many reasons to believe that Snape is evil — his background as a Death Eater, his close relationship with the Dark Lord, his making of the Unbreakable Vow, his general unpleasantness with Harry, and of course the cold-blooded murder of Albus Dumbledore. However... there are also a number of counter arguments that have been presented over the last months. Snape supporters are quick to point out that Snape could have killed Harry at any time, and in fact has saved Harry (and other members of the Order) on several occasions. They point out that there must be something that Dumbledore knows that no one else does.
I submit that this is probably true. Snape, by his own admission, spun a tale of deep remorse to gain Dumbledore's trust. We know that his remorse was about the Dark Lord's interpretation of the prophecy. But why the sudden change of heart? His detest for James Potter is well documented.
I hypothesize that Snape was secretly in love with Lily Potter. The latest book has revealed that they had much in common. Potions and their Half-Blood heritage, in particular. We saw that Lily was one of the few people to come to Snape's defense. And while we saw Snape respond negatively to her help, we know that boys mature — James was being a bit of a twit then, too. James settled down, and Snape fell in love. And we know from an interview with Ms. Rowling that Lily was a bit like the Ginny of her year — very popular with the boys. So it isn't unlikely that Snape had a secret love for Lily.
So why would Snape betray her to the Dark Lord? In an interview I read recently, an astute observer asked Ms. Rowling why the Dark Lord gave Lily a chance to live, if she would only surrender Harry, and asked further if the Dark Lord really would have spared her. Ms. Rowling said that he would indeed have spared her, and that the "why" was a very interesting question. I hypothesize that Snape bargained with the Dark Lord for Lily's life, perhaps feeling he could comfort her in her loss of husband and child, and maybe convince her to start a new life with him.
So many Snape supporters would agree that this is precisely the sort of circumstances that would send him straight to Dumbledore. They explain away Snape's behaviour as it is explained away in the book, except for the murder of Dumbledore, which of course has not yet been explained. The prevailing theory of Snape supporters seems to be that Dumbledore and Snape had agreed upon this end in advance, and that Dumbledore asked Snape to kill him, in order to save Draco, and to once and for all convince the Death Eaters that Snape was truly one of them. And that no harm was done, because Dumbledore was dying anyway, as a result of the potion in the cave.
I would like to offer another alternative. The Slytherin alternative. Snape is neither good, nor evil. Merely a man seeking an end. And that end is revenge upon the Dark Lord for killing Lily. To exact this revenge, he knew he had to remain close to the Dark Lord, but at the same time, covering all angles, he allied himself with the one man who was most likely to defeat the Dark Lord — Dumbledore. Snape has no true allegiance to either side. His one goal is the defeat of the Dark Lord, by any means possible.
And besides, what true allegiance could he have to the Death Eaters? While he fancies the Dark Arts, he realizes that blood purity is nonsense. And the Death Eaters would destroy him if they knew his background. But Snape has patience — he realizes that the Dark Lord has protected himself using powerful magic, but I don't think he's yet discovered what Dumbledore and Harry know. Snape knows that he cannot simply kill the Dark Lord.
I have not yet decided what will be revealed in Book 7. Harry has a lot of growing up to do before he can take on Snape. It's unfortunate that Harry is motivated by revenge — I don't believe that Dumbledore would approve. More musings will of course follow.