Superman once said that superheroes who kill are almost like weapons.
One of the strongest supporters of the no-kill policy has always been the Man of Steel.
He hates the idea of taking another person's life and understands the moral ramifications of having to, even if he is able to find a way around it.
Batman/Superman: World's Finest #24 by Mark Waid and Dan Mora reveals how dangerous superheroes can be with the next version of the no-kill rule introduced by Superman.
During Batman and Superman's journey to the Kingdom Come realm, Superman meets up with Boy Thunder, his long-forgotten sidekick, who turns out to have been Magog all along.
Eventually, Darkseid makes an appearance to look for Gog, which forces Magog to take the one action that could possibly drive Darkseid away: murdering Gog.
It works, but all the heroes around him chastise him for it.
Specifically, Superman expresses disappointment, saying that a hero who murders is just like a living, breathing weapon.