Yeah, I think we discussed something of a similar topic years ago and the question came up if Batman can't kill, then can he paralyze, maim or lobotomize? Grant Morrison had Batman do some pretty questionable things in his Batman Inc. run.
However, at heart, what is the intent behind the question? What is the framework or context against which it makes any sense? With fictional heroes, of course, they can kill and have killed, but the answer to the question "should" they kill can really only be determined by the specific story itself. With Batman and Superman, there is an added point or question. Is it necessary to their characters that they vow not to kill? Even if they vow not to kill, and avoid it throughout the story, the authors could still choose to have them kill at some point for dramatic effect.
Of course, the final answer depends on the reader. Some may believe that it is imperative that Batman vows not to kill AND actually manages to never kill on any adventure. Some may accept the vow to never kill but also still accept it if he kills in a story due to the circumstances of the adventure. Others can obviously accept a Batman who kills whenever necessary or even one who goes around with the intent to kill people who deserve it in his eyes. But that's more about the tastes of the reader than the fictional character of Batman (or, more accurately, the corporate property of Batman).
However, the question should fictional heroes kill has nothing to do with the question should real people kill in real life. A little earlier Arjan posted "there is a kind of person that really doesn't deserve life." And I agree... in fiction. In fiction, there are villains who can be portrayed as so malevolent that they must be killed. In real life, it is unlikely to find anyone who's done something so heinous that any one of us could not conceivably not have done the same or worse in their circumstances. Reality is messy, confusing and hurtful, which is the primary appeal of fiction as it is actually the result of a design, has a structure and delivers misery, death and misfortune to people who are not real so we can enjoy their suffering and, in many cases, an eventual triumph that most of us will never experience in real life.
Fiction is not a simulation of reality. Questions of what characters should do or intend to do in fiction are not directly applicable to real life. Even when they are meant to be such as discussed in this NPR story yesterday: http://www.npr.org/2016/04/01/472584700/when-it-comes-to-talking-sex-young-adult-books-can-be-a-parents-best-friend - it would be a mistake to say they provide a guidebook to dealing with the same or similar problems in the real world. No one would say "well, this character in this book is in a similar situation to yours, so why don't you do what they did because it worked out really well in the book."
And with super-heroes (pulp heroes, action heroes, adventure heroes -- all the various subgenres under the "Weird Fantasy Fiction" genre), the intent is 180 degrees opposed to the intent of any kind of moralistic narrative. Superheroes are a complete escape from the world - power fantasies. Even if they intend to be more realistic (like The Ultimates or Dawn of Justice) they aren't really realistic at all. They may have a more serious take and include things that seem to be more applicable to "the real world" (like cable news reports and heroes with Twitter accounts), but they are still filled with conventions we accept even though we know they are completely unbelievable.
Take a look (click) at this great page from the upcoming continuation of Millar/Quitely's JUPITER'S LEGACY.
Now, of course, Millar and Quitely have no problem at all with heroes that not only kill, but just plain murder the s**t out of people, but their stories are still primarily great superhero escapism with an edge...
If you look at the dialogue on the page, the swordsperson Raikou is a telepath who can control and confuse her opponent's senses. How do we know, because she tells them that during the fight. Also, she tells them that they are there to free Repro who is a superperson that can steal the powers of other superpeople.
Now, obviously, from a serious perspective, this is stupid. Why would she say any of these things during a fight? First, if her opponents don't already know how her powers work, then she's giving up an advantage by letting them know. Second, they should know why they are there, she doesn't need to tell them and, again, it's better for her if they don't know that she knows.
However, this is great writing because, as we know, the dialogue is actually for us, the reader, and on top of it, it has a bit of poetry in it. Repro is not just a superperson, she calls him a "human parasite." That gives us an idea of how she sees him and doesn't it just make you hate her a little bit more? She says "American Super Heroes" implying a very jingoistic view of the world. Also, she describes her powers confusing people's senses like "shuffling a deck of cards." Not a great metaphor, but a strong visual image to go with what we see her doing in the fight.
Finally, though, it is great comics writing primarily because it is something Millar excels at. It's a set-up for the punchline on the next panel:
So, you had this invisible robot character standing around while she beat up his allies waiting for the exact right moment to say his line when he could've zapped her quietly at any point? Yeah, it is stupid when you think about it "realistically," but it is great in the context of superhero comics.
Not that superhero stories are devoid of any sort of relationship to the real world, and there are stories that attempt to apply questions of superheroes using superpowers to political superpowers using their military might, but these stories are contained within the context of their world. They don't have such a direct moral connection to ours that the same questions can be applied in the same way. That what a real person should or should not do in the real world is just as applicable to what a fictional character should or should not do in a story. If it was, then our movies, comics, television shows and novels would all be as boring and confusing as real life.