As Snape left the grounds of Hogwarts, Harry couldn't believe Dumbledore even trusted him in the first place.
But what Harry didn't know is that Snape has indeed shown loyalties to both sides of this war of magic. Snape at first was a death eater, but turned away from that lifestyle and was trusted by Dumbledore.
However, his encounter with Narcissa and the Unbreakable Vow was merely a way of gaining her trust as to what the dark side is planning. He did tell Dumbledore this, and Dumbledore knew his only hope of saving the wizarding world was if Snape convinced the dark side he was with them; and killing Dumbledore while doing so. (I believe he will return on Harry's side in Deathly Hallows.)
The battle continues to grow fiercer between good and evil. J.K. has said that two main characters will die.
One definitely has to be Voldemort because otherwise there would be no meanings to the books other than the message that evil can conquer.
So, as the other character, I have decided the gullible Hermione Granger will be the next to die. Book smart and cleverness can't save her this time. I see Hermione as the character who believes she has figured it all out - but Voldemort himself has thought through this matter farther.
Now, even though we know two characters die, we don't know if any come back... Let's hope for the best.