A no killing rule makes sense IF the character can genuinely stop the stop the bad guys without killing them. Which, since the writers controls what happens in these fantasy stories, can indeed be the case. (In Joe Casey's run on Superman comics, he took it further and made Superman a literal pacifist. He never hit his enemies but instead found other ways to de-power/pacify/contain/etc. And it WORKED because these are fantasy stories and the writers control the circumstances.)
But that's not a sustainable plotting model. There will always be criminals / threats that cannot be easily captured, de-powered, never open to surrendering or negotiation (as in the movie examples posted yesterday) and will kill anyone for whatever motivates them. That does not leave the option for a non-lethal option to end to the conflict. If every villain ends up de-powered / contained / pacified, etc., there's no risk for the hero (or civilians), as the expectation would set in that the hero will always end his conflicts in the equivalent manner of a 1980s Saturday morning cartoon.