Batman and Joker are locked in a paradox. It's one of the most fascinating dynamics in all of fiction.

Batman won’t kill Joker because of his moral code; Batman refuses to kill, even though eliminating Joker could save countless lives.

Joker won’t kill Batman. Without Batman, Joker loses his purpose, his greatest source of amusement and challenge.

Batman needs Joker to give meaning to his mission. Joker needs Batman to validate his madness.