>>149681999In TKJ it's not even that Batman worries he'll never go back or that he doesn't want to kill if he has to, his main issue in fact is he worries he's going to eventually kill Joker in a situation he has to, but didn't need to.
He showed up at Arkham at the beginning to explain as much, but Joker wasn't there to hear the warning.
Batman only stops himself from rightfully beating Joker to death after a threat on his life to clarify things again.
So rather than suggesting any no-death rule, TKJ made clear if Joker doesn't rein it in one day he's going to just end up under the Batmobile when there's no time for his games at the expense of saving others.