DylanTheWise says....
*sigh*
It is quite obvious isn't it?

Snape told Dumbledore about the unbreakable vow that he made to Narcissa Malfoy and as to what he had to do.

Dumbledore understood that Snape had to fullfil this so he told Snape to go ahead with Voldy's plan.

That is what Hagrid overheard them arguing about.

Snape didn't know when or how it was to be carried out, but needless to say, he had an idea. Which is why he wanted to help Malfoy so badly, so that he would be in on the plan and warn the Order.

Snape didn't even know the death eaters were there until Flitwick came rushing into his office. Snape only stupified Flitwick to pretect him seeing as he is an old little dude and wouldn't last long against say; Fenrir Greyback?

Dumbldore was already doomed from drinking the Draught of the living dead and he knew that he would eventually die during a horcrux hunt anyways, having farked up his hand. If he were to lose a limb per horcrux, just think....ugh...

Snape is good, a complete jerk, but good...

That's right JK, I am on to you. I know your secrets!

Will says....
I didnt like the little mexican kid. ANDDD... if anyone who said anything on that street to that guy is reading this: i have one thing to say:

there is SOOOOOO much more logic behind why snape killed dumbledore. It's not just he was in the mood to go out in the middle of the night and kill a week old man. snape (a) had no choice or (b) noticed draco was failing!!! and the unbreakable vow he made with narcissa is crucial: making sure snape will protect draco, well if dracos failing and if he fails he gonna die.... DUH.

i need to go into leakylounge a bit more....

original says....
hum... these questions are quite original... like i've never heard them before...