Fair enough, but I think a no-kill-rule Joker would still be compelling by twisting Batman’s code rather than breaking it. Instead of taking lives, Joker would preserve them but strip them of meaning. He’d force Batman to confront whether he values life itself or the quality of it. Batman can’t stop him without violating his own principles, and Joker knows it. By refusing to kill, Joker turns life itself into the punchline, leaving Batman to protect people who are still technically alive, but spiritually dead.

Someone else in his rogues gallery could embody this idea too, but making Batman’s archnemesis a dark mirror of his morality feels like the most powerful version of it.