I have had this debate with my friends a lot of time. The problem is what starts out as a philosophical debate about ethics and morality, people have to drag in things character can or cannot do, because of their "profession" that has nothing to do with morality. For eg its okay for Capt or WW to kill because they are soldiers/warriors, but not for Superman because he is not...but isnt he exactly that when he is fighting to save the world/city? So the morality argument turns into different rules for different folks.
Another thing that is problematic is these type of discussions turn into movie specific debates that has been done ad nauseum.
My take as I said before is an emphatic yes. The way I view it is its a moral thing. Heroes dont go out to kill as a first resort. Its the very last. Try not to kill them. If you cant save innocents, or are putting innocent lives at risk for them, do the necessary thing. If you hold a holier than thou attitude, and that results in innocent people dying, YOU ARE RESPONSIBLE. You are not a hero. Which is why police officers can kill when necessary. You are right, these heroes are not the law, but thats what vigilantes are. When you become one, the same rules should apply.
Its a very subtle thing which very few stories can correctly address.Which is why when stories like Injustice ring hollow for me. Batman holds onto his sanctimonious attitude, when logic speaks otherwise. Its a stupid thing to say if he kills once, he will keep killing. Thats a slippery slope fallacy. Batman is a smart guy, he should be able to judge which situations need lethal force, and which dont, and if he cant he should hang the cowl. And Superman who starts as a kill when necessary attitude, soon becomes Punisher-esqe and starts killing willy nilly and goes into the world domination thing. Both extremes, no moderation. The same issue I have with Action comics #775 as well. I hope to read a story one day(Recommendations are welcome), where heroes/villains dont go to one extreme with these ideals, and actually do the sensible thing, and dont fall prey to the whole taken to the extreme characterization.
Well thats my elaborate thoughts on the matter. TLDR : Yeah kill when necessary, but use proper judgement.