Harry thought he would always remember the first person he killed.
He forgot.
Because the first blended into the second and third which mixed into the tens and twenties which blurred into hundreds.
The one Harry could never forget, was the last.
It wasn't on the battlefield, nor did it happen during the hundred years of unrest that followed.
Callused black hands comforted and held wrinkled white hands steady.
A small please slipped from Harry's mouth, but the man on the bed shook his head. Please, Harry. I need this.
Tears glistened in Harry's eyes and three slipped before he could wipe them away.
Then he said the spell he swore would never pass his lips again.
Avada. Kedavra.
The tombstone read:

Blaise Zabini
Beloved