Maybe Batman doesn't kill anyone because then the criminals will stop coming back and his life will have no purpose to it. All those years he spent brooding and struggling to become *THE* Batman would become meaningless. A part of him wants them to come out. Sometimes I wonder what if he helps them to escape the asylum. I mean how can people just waltz out of the high security place like Arkham.
I'm not saying he's bff with Joker, but He wants him to be the arch nemesis. Maybe because he's easier to control than rest of the rouges. He can beat up Joker anytime.Hes super smart and super manipulative. He can make Joker escape without even making Joker realize that.
He always keeps himself distant from the Justice League.
He has plans to kill the League because in case any one comes to know of his hideous acts.