The no kill rule still makes comics fans seethe

I mean the "actual" reason is that if superheroes existed they'd be an arm of or seen as being part of law enforcement i.e. the executive. The actual judgement of crimes and dealing of punishment would be, as is now, left to the courts as you wouldn't want people murking criminals extralegally no matter how justified. The real question is why is the Joker never executed for his crimes by the courts or, presuming that the death penalty is outlawed where Gotham is, why has no one cleaned him out ala Lee Harvey Oswald while in custody?