I think if anybody is going to off the Joker, it would be Gordon. While the act itself would be incredibly cathartic to witness, there’ll be repercussions.

Oh hi, Dr Wolper, is that you?

However, I think the reason Bats does not off the Joker, is because that is the one thing that separates him from the very thing he fights against. It’s almost as if Batman stubbornly, and ideally, hangs on to this one rule as a form of discipline or self-policing.

Maybe this is a bit of a romanticized way of looking at it, but I think it’s been brought up in several of the stories (you mentioned The Killing Joke, for example).

I think with the kill files, like you said, it would also include himself, should that line ever need to be crossed.

Of course, that creates the argument if whether or not the Joker NEEDS to be killed. And why is it Batman’s responsibility to kill the Joker?

Is it not reflective of the broken system, which allows the Joker to remain alive within itself?

I think if Batman would ever need to cross that line, he would do it, of course, but not without compromising himself. Whether he kills a hero, or villain, etc would be irrelevant (again, just like you said).