>>149875910
The meta reasons outweigh the in-universe reasons for why he doesn't kill. It's simple, his rogues gallery is as big as him, and killing the Joker is almost equivalent to killing Batman himself.
Take the Pegnuin for example. The tv show about this Batman side-character was better received critically and among audiences than the main movie itself.

I just think the contrast between a hardened criminal who killed his brothers when he was a child, killed a mother and son by setting them on fire, has deep rooted psychological issues with his mom to the point he decided to keep her a vegetable against her wishes, chose to torture his nemesis by sending her to arkham to rot instead of mercy killing her, killed his number 1 ally who he trusted with his life so they couldn't use him for blackmail and supergirl and krypto having adventures is too much