There’s a practical reason, from a storytelling perspective, why neither Batman nor Superman kill people.
What’s important here is that this is not a REALISTIC reason. Realistically, Batman and Superman would probably have to kill people if they were going to fight crime. It’s just going to happen now and then, even if they went out of their way to avoid it. But realism is always a sticky wicket when telling stories about invulnerable flying aliens and bat-themed vigilantes.
Also, though it’s usually a weak excuse to site story necessity as why something happens, every story has those elements that exist simply because they help the story be more interesting. From its most basic perspective, story necessity is why Batman doesn’t have a heart attack while sitting on the toilet or the Joker isn’t shot in the back by a no-name police officer. It is certainly realistic, but it isn’t satisfying.
There are some solid in-story justifications for why Batman and Superman don’t kill people, but there’s also a very strong pragmatic storytelling reason.
For Superman, his desire to limit the damage to the world and to protect as much life as possible places an extra obstacle toward his success. A Superman who is willing to kill, who views casualties as a necessary evil, is basically unstoppable. He might feel bad about killing his enemies or that innocent people are hurt, but he still wins. He’ll always win. A Superman who sees even the loss of a single life as a failure might be able to beat the bad guy, but he’ll have failed in his goals. And given his incredible powers, it would indeed be a failure.
For Batman, it’s even simpler. A Batman who kills would have no rogue’s gallery. People love Batman’s villains, but in order to justify the continued existence of those characters, you can’t have a Batman who is willing to kill criminals. This is a pretty central problem I have with BvS. A Batman willing to kill Superman on the off chance Superman might one day turn evil has no reason not to kill the Joker, who is undeniably evil.
Note that the Tim Burton version of Batman is fine with killing people, and he’s more than content to kill his villains, which he does.
If the new DC movieverse includes the new Suicide Squad movie, there’s absolutely no reason the Joker should still exist. Most people won’t care about this, of course, but these characters all live in the same universe. A Batman willing to employ lethal force would make short work of his famous rogue’s gallery.
There are problems with either depiction of Batman or Superman. Some people argue, convincingly, that by refusing to kill the Joker, Batman has enabled him to terrorize Gotham over and over again. In the amazing Justice League: The Animated Series, an alternate reality where Lex Luthor confronted Superman with this possibility created a universe where Superman went too far in his quest to protect the world.
But there are also scary connotations toward heroes that casually take life.
The answer isn’t black and white, which is why I tend to like the Marvel movies more. There’s no question that Iron Man, Captain America, and some others do kill during their adventures. Yet they tend to employ deadly force with reluctance, tend to seek to minimize damage, which is probably about as realistic as you can be in a cinematic media translation.
None of this will actually matter to most anyone. People who love the idea of a violent Batman will, ironically, be a fan of ultraviolent Joker. The contradiction this creates won’t matter. And it probably shouldn’t.
I think about this stuff from a writer’s perspective. And that perspective says you can’t have angry, violent Batman and angry, violent Joker sharing the same space.
And a Superman who is so carelessly destructive that he causes cities to crumble isn’t one who will be loved. Which would be fine if BvS didn’t seem to want him to be both feared and loved simultaneously.
Yeah, none of this matters, but it matters to me. It’s why I don’t mind a violent Batman or Superman, but I do mind a universe where the true aspects of those characters are not explored beyond the most shallow manner.