In Gotham, carrying out justice without killing was an extreme test of one's mental endurance. The Bat-Family could do it because they hoped criminals would repent amid the suffering of the rest of their lives.
But some people did not see it that way. They followed the old mob code of an eye for an eye and blood for blood, merely in the name of justice.
Perhaps if they moved to another city—Star City or Seattle under Green Arrow's protection, Wonder Woman's London, or Aquaman's Mercy Harbor—there would be no problem. Even if they killed someone, those people would not trouble them.
Bad people deserved to die. As long as the bodies were cleaned up and no one was frightened, it was fine.
But not in Gotham. Here, the only rules were Batwoman's rules.
Batwoman preferred criminals to live through pain, to experience the fear she had once felt, and to spend the rest of their lives in torment of both mind and body.
Those in the Justice League who upheld a no-killing rule had their own reasons as well. J'onn did not kill humans because it could easily turn into huge news about a Martian killing an Earthling. White Martians already had a reputation not much better than Ghouls, so as a Green Martian, he had to be even more careful not to invite criticism.
Superman did not kill because he believed killing others would only leave darkness in his own heart. He lived in the United States and should act according to its laws. His power was not an excuse for indulgence; he should hold himself to stricter standards than ordinary people. That was responsibility.
Barry did not kill because he had originally just been a physicist. He had never even killed a lab mouse and could not