Here's the thing. If Batman kills the Joker, why would he not kill the Scarecrow, or Black Mask, or Mr. Zsaz (assuming he's not already dead) etc.? I get that he wouldn't necessarily become an uncontrollable killing machine, but what limiting principle would exist whereby killing the Joker is justified and permissible, but killing half of his other rogues gallery (who are serial killers) isn't?
I don't see anything wrong with that either. Batman positions himself to be in conflict with terrorists and mass murderers. He'd have an excuse for not killing them if his conflicts with them would actually make their sprees stop, but they don't. The most Batman ever seems to do is prepare to put them away once they've escaped and committed more murders. It's just ridiculous that his writers insist on how intelligent he is while at the same time insisting that he's incapable of telling the difference between Hitler and a dude that ticks the wrong box on his taxes.
Okay, well that would make him a murderer. He has no authority whatsoever to determine who is and is not worthy of life. He is not a judge. He has not been elected by the people to pass such judgment. Batman is not a murderer, and I prefer him that way.
This has always been a pretty poor argument to me to. If dude cared about respecting laws so much then he wouldn't be a vigilante that routinely goes to war with the cops. At best he'd be a lobbyist or someone that contracts his surveillance tech out to the government. But he doesn't do any of that. He goes out at night in a mask and fucks people up, commits dozens of crimes each night, and refuses to place himself under any oversight that he can't easily undermine. Even in joining the Justice League he only does it only so that he can be the final authority over right and wrong, where anyone that disagrees with him can be subject to one of his contingency plans. If he was gonna blink when confronted with the reality of being judge, jury, and executioner; he shouldn't pride himself on how effective he is at being judge and jury.
Hmm. Your argument seems to boil down to "Why be a control-freak vigilante if you're not also going to be a murderer?" There is a finality to killing which differentiates it with the other crimes Batman commits (aggravated assault etc.). Batman does not want to be a killer, because he thinks that if he were, that's all he would ever be. That's not who he sees himself as. ---