Hmmm Snape, /sigh I wish to think that he's a good guy, BUT, I could definetly see her making him a bad guy, purely to point out that good ppl, ie Dumbledore, make mistakes. I don't believe Dumbledore is still alive either, I think he's really gone, 'cause remember his "sleeping picture" had appeared in his office.
Well, I just feel that she had Snape "kill" Dumbledore to make Harry hate him even more. But then he'll have to trust him in book 7 to get to Voldemort, that seems to be the level of depth that she puts in her plots. (now don't get me wrong, I do like the books a lot, but they are children's books and even for kids they are rather simplistic imo, not a bad thing but just an observation)
I'm not really sure about Dumbledore being dead or not, I doubt it a bit but if he did die, then it was a plan with Snape involved to bring down Voldemort in the end.