“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.)
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.)