Batman - Because Killing Them All is Not an Option

It's not quite that simple.

Batman's refusal to kill despite everything is perhaps the single greatest defining aspect of him as both a superhero and a character. This absolute moral bedrock is what allows him to dwell in darkness without actually becoming part of it.

They actually covered this at the end of that one animated movie, when the Red Hood asked Batman why, despite "all the graveyards he's filled", he insists on sparing The Joker.

Batman replies that if he killed him, it would essentially be the ultimate compromise of who he is, and that he'd "never be able to go back".

Batman also reflects on your point during Arkham City, but I won't say anymore in the event you haven't played it yet, in which case you should. Like, right now.