My main reason for this decision comes from my AP English toolbox. Throughout the entire series, Jo has used narrative misdirection. Whatever Harry thinks of a character, we think too. What reason do we have not to believe Harry? He hates Snape, so we hate Snape...
Which is exactly why we should trust Snape. Harry has blinders on when it comes to people. He immediately likes a person or doesn't, and nothing can change that. No matter how many good things Snape does, Harry is determined to hate him.
It is true, Dumbledore is not all-knowing. As he himself said, since he is so wise and so trustfull, his mistakes tend to me much bigger than normal peoples'. It does seem foolish of Dumbledore to trust Snape so fully, even with all the evidence against it, unlesshe knew something we didn't.
I am of the opinion that Dumbledore and Snape had arranged for Snape to kill Dumbledore. Dumbledore does not want Malfoy to go all the way to the dark side, so he wants to spare him killing. The conversation between the two, overheard by Hagrid, confirms this theory, as Snape is overheard telling Dumbledore that he doesn't want to do "it" anymore. "It", I believe, is to kill Dumbledore.
I think that Snape will untimately redeem himself my sacrificing himself, thereby bestowing Harry with protection similar to that his mother gave him. Harry will then be able to defeat Voldemort.
Next pensieve: Harry's Fate