As for you sankydreams,did'nt u read the 6th book?

 

Snape had made the Unbreakable vow to narcissa infront of Bellatrix

that he would protect malfoy.

 

He had sensed that fenrir might kill Draco(who listens to dumbledore's final talks and decides to turn a new leaf)and so he killed Dumbledore(or else Snape would be himself killed for breaking the vow)!