A lot of people misunderstand Batman’s no-killing rule and think that it has to do with his belief that anyone is capable of redemption. While this is a factor, it isn’t the main factor, which is that Batman believes that once he crosses that moral line he won’t be able to stop.
People have this impression that he could just snap the Joker’s neck and then go back to tossing the other rogues in Arkham or Blackgate. But instead Batman believes that if he could justify killing Joker, he could justify killing Scarecrow, and Riddler, and Penguin, etc
And if he can justify killing off the rogues gallery, why not mobsters as well? Why not corrupt cops? Why not white collar criminals? Ultimately Batman would become a worse criminal than anyone in his rogues gallery.