yeah, like i said, it's starts to become a difficult argument to make, because we want to argue in terms of batman and joker being real people and they aren't. in the real world neither character can exist. they are monoliths and paragons and they just can't survive in the real world.
batman is the definition of gray area. he maims people, breaks bones, tortures them... as a reader you want to look at it and say that anyone who does that kind of stuff SHOULD at some point start using a sliding scale on what's okay and what's not. it's INCREDIBLY easy to justify joker's death vs hundreds or thousands, but again, batman doesn't think that way. he can't, because as Big Toe puts it, batman is just as insane as the joker. might honestly be even worse. and honestly? batman probably knows this as well. if he budges even an inch on his stance he'll be off to the races.
batman doesn't not kill people for some altruistic "killing is bad" reason. batman doesn't kill because it's the ONLY thing separating himself from the ones who murdered his parents. he doesn't kill people for HIM. he doesn't dress up like a bat and fight bad guys to save the world. he is 100% selfishly acting out. gotham is HIS city. he says that all the time. in his eyes, gotham doesn't belong to the people, it belongs to him and he gets to do whatever he wants in that city. and he WANTS to hurt people.
