I hate to be a wet blanket, but all these discussions ring hollow for me because these types of stories are inherently contrived. In the real world, not only would someone like the Joker not exist, even if they did exist they would be put away for life in a maximum security cell where escape is impossible. He wouldn't have the benefit of a convenient plot that allows him to break free whenever and wherever. He would be contained, forever. So there's really no moral quandary there.
Batman's universe is built on manufactured incompetence (by law enforcement). Villains escape because they HAVE TO escape for there to be a story. Batman's dilemma would not exist without that fabrication. In the real world, the actual dilemma is not pragmatic but emotional, whether there should be state-sanctioned revenge killings or not. A much less interesting and (in my opinion) much more straight-forward debate. Again, I understand I'm being a joykill, but I get annoyed at people who try to apply the logic of fiction to real-world scenarios, when it simply doesn't follow.