What was our once-beloved Severus thinking when he cast that curse upon the man who gave him opportunity and love?
The Unbreakable Vow was upon him. It was either do it or die. He heard what were to be Albus Dumbledore's last words, "Severus, please. . . ." Yes this man had put all trust into him, and helped him achieve a life which could not have been gained without Dumbledore's assistance, but then again the past couple years of using him, like he was a toy, using him for information for his own victory.
Suddenly Severus felt rage creep through his veins. 'Why kill myself when I have been nothing to this man, but an educator to fill a spot and a traitor. He'll die now even if I don't do it anyways! So why risk my neck when his death is unavoidable? I have a good seat next to the Dark Lord, and without Dumbledore we will have victory.'
With that, Snape pointed him wand at Dumbledore's chest and said "Avada Kedavra." No later had Dumbledore fallen to the ground in a slump, and he and the other Death Eaters were running out of Hogwarts.