Oh man, this reminds me of what I did in my game with a paladin who indiscriminately killed everything, including a defeated, surrendered goblin.

As he made the killing blow, I gave him a vision of the goblin's life, from the goblin's point of view, culminating in seeing this glorious vision of a holy paladin and his regret and guilt over the life he has lived so he surrendered to the paladin who...stabbed him through with his greatsword.

The Paladin had a "Gleaming" tabard (i.e. was always clean, dirt wouldn't stick on it). But it was forever after stained with the bloody handprint of the goblin.

It was surprisingly emotional for the player. He just sat for a bit looking down at his character sheet and said, "I hate you." But his character became amazing after that.