Re: If the author wanted a "killing is always wrong" shpiel, he picked the wrong villains.
I don't care what excuses are given, the Joker gets out and kills again, time after time after time. It's his pattern. No matter if he might be redeemable, no matter if he's not legally responsible for his actions, not killing him directly results in more innocent deaths. The only responsible thing to do is put him down like the rabid dog he is. And failing to do so while knowing what the result of inaction is renders one partially culpable for those deaths unprevented.
In failing to end the Joker's murder spree once and for all, he's responsible for the deaths he willfully chose not to prevent. There's no moral justification for that.