Okay, so we've all heard basically to sides to the Snape issue so far. First is the obvious one: Snape is evil, he's been working for Voldemort this whole time. By killing Dumbledore, he just showed his true colors. Unfortunately, this theory has some major flaws: Why did Snape save Harry's life in Sorceror's Stone, for instance. Why did Snape stun Flitwick instead of killing him, Hermione, and Luna? No, Snape is not evil.
So we get theory two : Snape is still a good guy, he had no choice but to kill Dumbledore and Dumbledore wanted him to. His job was more important than Dumbledore's. He was forced to make the unbreakable vow or else break his cover. Frankly, I think all this is wishful thinking. I think if Snape was truly loyal to Dumbledore, he would've chosen to break his cover rather than making a vow to kill him or die - especially if he could've taken out Ballatrix LeStrange in the process.
No, I choose option three - Snape is out for himself, and no one else. At the begining of Half-Blood Prince, Snape has carefully manuevered himself into a position where he is trusted by both Voldemort and Dumbledore - so whoever wins, Snape wins. As long as he keeps walking this fine line, the outcome of the war doesn't matter to him. From his perspective, he's in a really good spot. Then something unexpected happens - Narcissa asks him to make the unbreakable vow. Suddenly, Snape is forced to pick a side. Then he realizes that if he chooses to make the vow and Draco succeeds, he may still be able to choose to work for the order. So he chooses the option that gives him a chance, albeit a slim one, of maintaining his neutrality.
Then, on the tower, Snape has a choice - kill Dumbledore or die. Since he's been out for himself this whole time, obviously he chooses the option where he survives.
Snape is a Slytherin - as the sorting hat says "these cunning folk use any means, to achieve their aims," - sounds like just the kind of person who would play both sides for his own personal gain.