Killer heroes?

I've just spent a day watching DC animated films on netflix and got to thinking about the nature of super heroes, villans and the moral codes they go by.

One of the films I saw had Superman encounter Manchester Black, and the whole story revolved around Superman not killing the bad guys where as Black would.

Most Heroes don't kill in the comics.
No matter how many times Batman beats the Joker, Joker always comes back and kills more people. Lex Luthor, Red skull, Magneto... Bad guys who kill innocent people, cause much suffering again and again, only to be "defeated" and locked up where they can come back eventually to kill again.
The Heroes for whatever reasons, don't kill the bad guys despite the fact that letting them live inevitably causes more death and suffering.

I get the reasons for letting bad guys live from a story telling point of view; the Joker needs to be alive as an ongoing arch rival to keep the story going.

But what if we had there Super Heroes and Villans in the real world?
What would be the arguments for and against Heroes killing the bad guys then?
If we did have the Joker, Lex Luthor, Batman and Superman in our world, would it be ok for the bad guys to be killed by the heroes? Would it be irresponsible of the heroes to not kill them?
Or could there be another alternative?

the difference is a moral one, if the hero kills their enemies then essentially they are no different. Careful when fighting tyranny lest you become a tyrant.

I think that in the end, it would be one or the other, as it always is. One of them is going to have to die, and the hero would have to kill the villain to end it.

I think in the real world the villains wouldn't be able to escape their lockups to kill again. Them getting free is also a comic book story driven point.

I'm... trout I'm actually with Steve on this one.
Damn.