Black and white. Shades of grey.

Crime is black and white. Guilt is grey.

Sherlock sees the truth. Usually it leads to black and white, convictions. But sometimes it's grey. Then he doesn't know what to do. He both hated and loved those. Sticky things, where what was right and what was right contradicted each other. He usually trusted John for help in those situations.

John. A soldier with strong moral principles. (Evident by the fact that he waited to shoot the cabbie until Sherlock was in immediate danger.) Had been to war. Killed people. ("You wanna remember, Sherlock: I was a soldier.I killed people.")

Seemed fine about the whole thing; told everyone he was fine, didn't want to talk about it, smiled. But Sherlock could tell he wasn't. Knew that every time he took a life, even if it was an awful one, like the cabbie's or the rapist they'd been tracking for weeks, that John had to shoot in the end.

He claimed he was fine. Sherlock could see six things that proved he wasn't.

The decision was easy. Living with it, that was what was hard.