He'd known this day would come, of course. Known that the inevitable would happen and he would have to keep his promise (not the one to Narcissa. The prior promise.).
Nonetheless, standing on the tower, staring at the Headmaster, he found it difficult to raise his wand at all. Dumbledore looked old, broken, wearier than Snape had ever seen him in sixteen long years.
"Severus, please…"
Snape felt his face twist with fury and revulsion, knowing that this was the point there would be no turning back from. He thought of Albus asking him to do this, and the anger for his insistence was enough.
"Avada Kedavra!"
He didn't turn away; let his mouth twist in disgust as the greatest wizard he'd ever known fell, and was half surprised that he didn't float to the ground like a feather.
And then it was done.