There was a small part of Snape that wasn't repulsed at what he was to do, a little part of him that was enjoying this, just a little.
He had been under control for so long. First to Lily, and then forever to her memory.
Lily, the angel of Snape's childhood. Her son was probably here as well. Soon Harry would think that Snape was just as evil as ever. It wasn't true.
"Severus…please."
Snape stared at the old man who had given him life after Lily's death. Hatred and repulsion for what he was to do grew on his face. He hadn't killed anyone by his own free will in 16 years. To do so now, especially Dumbledore, was almost…difficult.
Snape couldn't explain why he wasn't totally repulsed by the idea of ending the old man's life. Perhaps it was because Dumbledore reminded him constantly of Lily, or perhaps because once he was dead, Snape had only himself to turn to. He had been under Dumbledore's service for so long. Snape longed to once again be free.
But Lily, she would have hated this.
"Avada Kedavra!"
And the last order Albus Dumbledore ever gave Severus Snape was done.
As Snape grabbed the Malfoy boy, turned to run, knowing his life was in danger, he caught a glimpse of a pair of eyes shining in the darkness of the tower. Brilliant, green eyes. For a second Snape was engulfed in guilt for the life he had just taken, cowed under those ferocious eyes of hers. But then he noticed that the eyes were full of hate, disgust, so like Lily's eyes had been after he called her the unmentionable name. He never remembered those eyes, except in his dreams. He didn't want to. It made him too disgusted with himself.
They were now Potter's eyes, no longer did only Lily's eyes glow that green. The eyes were filled with an enduring hatred.
And so, as Snape ran from the spot where he had killed the man who had protected him for nearly 16 years, it was with the eyes of the only person he had ever loved etched within his memory once more.