Snape is an enigma. What is he doing? From the very first book it has been made incredibly clear that two things are true. Firstly Snape hates Harry as much as anyone can hate; and secondly, Dumbledore trusts Snape implicitly. So when, at the end of the sixth book, Snape kills Dumbledore and runs from the chance to do the same thing to Harry everything we thought we knew got flipped on it's head.
Is Snape Good or Evil?
The question everyone is wondering. Is Snape really evil? After all he did kill Dumbledore, was Dumbledores death an elaborate hoax, or did Snape plot with Dumbledore to make his death a reality.
"I was comin' outta the forest the other evenin' an' I overheard 'em talking -- well, arguin'. ... I jus' heard Snape sayin' Dumbledore took too much fer granted an' maybe he -- Snape -- didn' wan' ter do it anymore ... Dumbledore told him flat out he'd agreed ter do it an' that was all there was to it."
Snape and Dumbledore were planning something...
...somebody else had spoken Snape's name, quite softly. "Severus..." The sound frightened Harry beyond anything he had experienced all evening. For the first time, Dumbledore was pleading. Snape said nothing, but walked forward and pushed Malfoy roughly out of the way. ... Snape gazed for a moment at Dumbledore, and there was revulsion and hatred etched in the harsh lines of his face. "Severus... Please..."
The most common theory, and the one I believe, is that Dumbledore is pleading with Snape to kill him, not to rescue him. He knows that by dying he can save the lives, if only temporarily, of Snape, Malfoy, and Harry; all of whom are bound to have things to do to rid the world of the evil of Voldemort.
Dumbledore has said time and time again that he trusts Snape, yet everyone thinks the reason for this is explained in Book 6, when we find out that Snape told Dumbledore that he had overheard part of the prophesy. To me this seems like a very weak reason and I feel there must be more to it that that. For all Dumbledore knows Voldemort suggested Snape tell Dumbledore what he knows in order to get someone on the inside. It's this that is the biggest mystery around Snape, who IS he working with? Is he good or bad?
Half Blood Prince
You've read the book, you know Snape is the Half Blood Prince. Does anyone else think the reveal for that was rather weak? It all seemed very rushed at the end with Snape 'rescuing' Harry and Malfoy, and telling him who he was. Of course it fits fine and I don't doubt that Snape is the 'prince' but it was a bit of a let down the way it was written. Maybe it will make more sense in future re-reads.
Dumbledore is not Dead
Some of it is a bit far fetched but I agree with a lot of what is said on the Dumbledoreisnotdead.com page about Severus Snape. As I mentioned before I think a lot of the other content on this site is silly, but there is a reasonable amount of believable argument on the Snape page.