Here's a fun and widely-known fact: Superman and Batman don't kill, because Batman saw his parents horribly murdered in front of his eyes and Superman's a good guy about stuff like that. That's why the Joker and Lex Luthor are still running around: Superman and Batman just do not kill.
Oops. Except vampires, apparently. And intelligent machine entities. And monsters, space-dwelling sentient clouds and the occasional alien. Superman and Batman don't kill humans. Which is bullshit. It's the same sort of logic that shows up in fantasy novels all the time, where the hero spends most of the narrative carving up hopelessly outclassed members of supposedly 'evil' races like goblins only to get all moral and hesitant when his (or her, but usually his) opponent is another human, no matter how demonstrably evil. To me it smacks of crypto-racism - I'm sure that if I were still in university I'd be gearing up for an essay that mentions the Other a lot (with maybe a hint of the ol' Male Gaze, just for variety). I mean, *why* do we not kill other people, when you get right down to it? Because they're self-aware entities, just like we are, and if that's enough to save the Joker from your Bat-Wrath then you shouldn't be so damn casual about ripping off those vampire heads, Bruce. And Clark.