I think that Severus did not want to kill Dumbledore, but he had to because of the unbreakable vow. Since Draco couldn't do it, Snape had to. But on the other hand, I still think Snape is a huge git.

I think the Marauders said it best when they said:

"Mr. Moony presents his compliments to Professor Snape and begs him to keep his abnormally large nose out of other people's business."
"Mr. Prongs agrees with Mr. Moony, and would like to add that Professor Snape is an ugly git."
"Mr. Padfoot would like to register his astonishment that an idiot like that ever became a professor."
"Mr. Wormtail bids Professor Snape good day, and advises him to wash his hair, the slimeball."