Personally I think he is but that there is a reason behind it. I'm a die hard fan of Snape and so I find it so hard to succumb to the idea that he is evil, I don't think he's as black and white like that. I think he and Dumbledore planned the death in part to prove to Voldemort that he is on his side. He would have greatly inhanced Voldemorts faith him him with the fact that even the other members of the Order knew nothing about his supposed betrayal. Also to keep hidden his identity as a double agent finding out things about Voldemorts next moves which could be crucial information.
Also I don't buy that whole supposed reason why Snape had switched sides which is given in the book it must have been something stronger than he felt bad about letting James and Lily die as Lupin said Snape hated them Dumbledore isn't stupid. When I first read that Snape was a traitor and killed Dumbledore I was inconsoleable I love both of them but I'm part of this brillaint communtiy called we_trust_snape and I have read so many theories by people who love Snape as much as I and I can't help but thing that there is so much
more to him than simple evilness. He has done cruel things in his days of being a death eater but I think he is genuinely looking for redemption
in any form of the word.
Also to bring it back to Dumbledore J.K. Rowling wouldn't have killed off an extremly important, intelligent, powerful prominant and sweet
character so early in the war if there wasn't a very good reason for it. My dad also reads the series and when I was talking to him about what
happened and he said that the war was like a chess game the Queen is the most useful piece but ultimately it is the king who can end the game.
Dumbledore is the Queen and Harry is the King despite the importance of Dumbledore he is, in this war expendable (sob) The war can be won
by Harry alone and Dumbledore knows that.
Before I heard that hypothesis on the matter I thought the wizarding world had no hope they'd my three favourite characters (I was so annoyed
at J,K Rowling at this) Dumbledore, Snape, Siruis and all the others before James Lily and Peter. And Voldemort now had control of his deatheaters
dementors and giants. Despite the odds being fiercely stacked against them I still think the goodies can win.