"Killing is wrong, no matter what," is a very powerful and standard meme for heroes.

It's also convenient for writers. Imagine what would happen to the Batman comic book series if someone finally got around to putting a bullet through The Joker's brain. (In a Discworld story, it's suggested that "heroes" and Dark Lords have a bit of an understanding: Dark Lords keep on making all of the mistakes on the Evil Overlord list, and heroes keep on letting Dark Lords escape after the day has been saved.)