(Image: Eduardo Ferigato)
Traditionally, the Joker is Batman’s greatest foe and the focus of hundreds of Batman storylines. There's a common sequence of events: Batman captures the Joker and sends him to Arkham Asylum. The Joker escapes and commits terrible crimes. The cycle repeats.
So why doesn’t Batman just kill the Joker and break the cycle? Quora members pondered this question recently. Jesse Richards, an artist and web designer, responded that Batman’s commitment to not killing intentionally—his self-control—is his superpower:
Because the Joker wins if Batman kills him. That's what the Joker wants. Everything he does is to taunt Batman into killing him. In fact, the interesting part of their relationship, the real conflict of each story, is not to see if Batman will stop him (he will), but to watch Batman struggle with not killing him, because anyone other than Batman would of course kill him. This self-control is Batman's superpower.
The Joker and Batman are each trying to prove a point to society - and really to us, the readers. The Joker wants Batman to kill him because he perfectly embodies chaos and anarchy, and wants to prove a point to everyone that people are basically more chaotic than orderly. This is why he is so scary: we are worried he may be right. If the Joker is right, then civilization is a ruse and we are all truly monsters inside. If the Joker can prove that Batman - the most orderly and logical and self-controlled of all of us - is a monster inside, then we are all monsters inside, and that is terrifying. The Joker is terrifying because we fear that we are like him deep down - that he is us. Batman is what we (any average person) could be at our absolute best, and the Joker is what we could be at our absolute worst. The Joker's claim is that we are all terrible deep down, and it is only the law and our misplaced sense of justice that keeps us in line. Since Batman isn't confined by the law, he is a perfect test case to try to get him to "break". The Joker wants Batman to kill a person, any person, but knows that the only person Batman might ever even remotely consider killing would have to be a terrible monster, so is willing to do this himself and sacrifice himself to prove this macabre point. Batman needs to prove that it is not just laws that keep us in line, but basic human decency and our natural instinct NOT to kill. If Batman can prove this, then others will be inspired by his example (the citizens of Gotham, but again, also the readers), just as we are all inspired every day to keep civilization running smoothly and not descend into violence, anarchy, and chaos. This ability to be decent in the face of the horrors and temptations present all around us is humanity's superpower, the superpower of each of us. The struggle of Batman and the Joker is the internal struggle of each of us. But we are inspired by Batman's example, not the Joker's, because Batman always wins the argument, because he has not killed the Joker.
I’ve never found this view convincing. Batman has the reputation for striking terror into criminals, particularly while interrogating them. It’s not just that he’ll turn them in to the police and they’ll go to prison. They’re afraid that he’s going to kill them. But if Batman is widely known in underworld circles for his unwillingness to intentionally take a human life, then why would they be so afraid?
Criminals easily and frequently escape from Arkham Asylum. This, too, must be widely known. So when Batman refuses to kill the Joker, he knows—or must realize eventually—that the Joker will soon be free to commit terrible crimes again. The only reliable means of ending the menace that the Joker presents to the ordinary people of Gotham City is to kill him. So killing the Joker, far from being monstrous, is a civilized act. In fact, it’s the most civilized choice that Batman could make.
There are schools of pacifism that may disagree with me on that point, but I’d find it hard to place Batman within any pacifistic tradition.