Especially since there have been occasions when Superman was willing to kill when faced with no other alternative, such as with the Phantom Zone Criminals Post-Crisis from an alternate universe who threatened Earth (unless Final Crisis negated that), and of course the usual superhero disregard to killing "monster" opponents.
It reminds me a little of a moment from the cartoon JLU, episode "THE DOOMSDAY SANCTION", where Superman zaps Doomsday to the Phantom Zone after barely surviving an assassination attempt from him, and Bruce Wayne gives Superman a tongue lashing about it, thinking it too "holier than thou". The cartoon's writers had Clark make a wisecrack and poo-poo the situation, so Batman could act as the veteran voice of dissent. A better bit of writing would have been to have Superman (or Diana, who was present) question Wayne's logic. "I agree, having a bunch of us decide to zap Doomsday to another dimension without technical due process wasn't very easy or ideal. But what was our alternative? Kill him? We don't kill. Hand him back to the same federal authorities who not only created him to begin with, but failed at every juncture to contain him? This isn't a portly fellow with an umbrella fixation, this is a man who is fully capable of killing even me if given half the chance and every time he escapes from jail, countries could perish. What was your solution, if not this, World's Greatest Detective?" Instead, Batman got to be a bad ass without having to defend his "just to complain" position, and a solid scene wasn't as good as it could have been.
But now I am mumbling.
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