Which heroes should and shouldn't kill is completely dependent on the hero. I've always seen Batman as a character with a pathological aversion to killing. Wonder Woman on the other hand sees killing as a last resort. Aquaman being a king is often put into situations were is actions more then likely will cause death. Superman is an interesting case to me, in most cases his powers are versatile enough that he doesn't have to resort to lethal force, but some of his antagonist are so powerful, that their very exitance places everyone in danger.