15The Joker can’t be executed because he’s legally insane
Jason Voorhees and Ted Bundy combined couldn’t match the Joker’s accummulated body count. The reason he can kill — and kill — and kill again with impunity is because he’s legally insane and not responsible for his actions. All Gotham City can do is lock him up in Arkham.
defense, however, doesn’t work like that. To escape execution, the Joker would have to be incapable of understanding things like “pumping someone full of Joker venom will kill them” or of knowing the difference between right and wrong. The Joker knows the difference, he just doesn’t care. And when he kills, he knows perfectly well what he’s doing. For the purposes of charging, convicting, and executing him, the law would consider him sane.
It’s possible the law is radically different in the DCU but to date Batman stories don’t demonstrate that.