And yet, civilians are allowed to kill in self-defense and in defending other civillians. Even cops aren't allowed to just run in and kill a perp, there has to be some kind of immediate threat to their own person or other people involved. I don't see why superheroes don't get that same right.
Can you imagine any jury who would have convicted Diana of murdering Maxwell Lord if they had known it was to prevent Superman murdering the one guy capable of stopping the OMACs? But no, the public wasn't given the context or the situation, it was just this mass outcry of "murder, murder, MURDER!" And even the superheroes who DID know, including Batman whose life she just fucking saved thank you very much, decided that she was some out-of-control homicidal maniac.
Other things that grate the hell out of me - Dick agonizing over nearly killing the Joker in Last Laugh, Dick endlessly flagellating himself for letting Blockbuster take that bullet, Batman being all chummy with Two-Face even after he'd once beaten the shit out of Dick when he was little, Batman saving the Joker's ass from the legal system in Devil's Advocate, Superman not killing Joker even to save Lois, Batman continuing to save Joker's ass...after a while, it becomes less about superhuman self-control and more about something inhuman.
This is why my favourite moments in comics include Selina shooting Black Mask dead and Jason Todd beating up the Joker seventeen ways to Sunday and confronting Bruce about letting him run loose, Helena Bertinelli dangling Yasemin by the throat over a ten-storey drop for threatening her students. Now those are supremely human and above all pragmatic reactions I can get behind.