Re: If the author wanted a "killing is always wrong" shpiel, he picked the wrong villains.
Because the Bat-clan values life above all else. Every one of them has thought about killing Joker at some point, but that's too pragmatic. They consider that the easy way out. For Batman (which he then teaches to his partners), he has to attain for a higher sense of justice. He fights for an idealistic vision of the world where you can't just tyrannically kill off someone you personally judge to be irredeemable, where--even when there's no evidence to back it--he has to believe that someone like Joker is still a life that you can't just decide to extinguish at will. If he kills, then he destroys the ideal that he's pursuing.