It’s a core fact of the Batman mythos that he does not kill. Not when he confronted Joe Chill or his son, not when Joker murdered Jason Todd and not even when he paralyzed Barbara Gordon and took her father hostage. It took Darkseid and his control over the planet and inevitable rule of the multiverse to make him break his rule. Admittedly, Darkseid didn’t actually die right then and there. His body perished but his mind and spirit wandered free (check out the Final Crisis series for more on that) so Batman didn’t actually kill him. Although there was no indication that Darkseid’s personality would survive, if anyone knew it would be the Detective. But it was only after countless heroes and villains and maybe several billion other people were indoctrinated by the Anti-Life Equation that Batman could bring himself to willingly end the corporeal life of another sentient being.
During the Final Night, Polaris assisted the heroes of the world in attempting to defeat the Sun-Eater as he realized he would also die if they failed. He set aside the customary greed and violence that powers most villains and tried to help the world. Although it did not last long, Polaris helped the forces of good. Several other villains have slipped across that line a few times in the past, like Poison Ivy during the No Man’s Land conflict or when Arkillo and Atrocitus aided Kyle Rayner in mastering all the colours of the spectrum. Captain Cold proved himself capable of change when he joined the Justice League with Lex Luthor. When they chips are down and the stakes high, heroes and villains alike band together to protect their various ways of life.
But then there’s the Joker.
The Crown Prince of Mayhem, the insane king of Arkham, this maniac somehow keeps getting lose and invariably kills whenever he can. Other than a potential formula for eternal youth that his spine apparently creates, Joker had done nothing that actually helps in the progress of humanity. He’s killed countless times, disfigured hundreds of others and once threatened to destroy New York and only relented when he realised that his own life was on the line. It has been argued that he’s insane and thus not responsible for his actions. Somehow, I doubt that somebody not in control of their actions has the ability to concoct Joker Venom or create such elaborate tools of torture and murder. Batman argues that the instant he pulls the trigger, the Joker wins. But surely there is no good reason to keep the Joker around. We are talking about a man who literally sold his soul to the Devil for a box of Cuban cigars.
During the Search for Ray Palmer, we are taken all throughout the Multiverse as we track down the tiny scientist with the main clue being a mark left behind in the shape of his logo. Several stops later, we reach Earth 51. Very little is actually revealed about this world but it seems to have similar history to that of the pre-Crisis universe with a few slight twists. Instead of becoming the Atom, Ray Palmer served the Justice League as a scientific advisor and had not met Jean Loring before the run-aRay arrived in that universe and replaced his recently deceased alternate. Jason Todd also met his untimely end at the hands of the Joker but apparently did not come back from the dead. Now we hit the most crucial difference between the two universes: Batman cuts loose. Realising the futility of his constant battle against crime, he takes up a gun and takes down the murderer of his partner and son. But it doesn’t end with the death of the Joker. Using his detective skills, he is able to hunt down and murder several other villains without any of his colleagues being able to prove it. Eventually, the rest of the killer community gives up. Most of the other heroes are able to go public with their identities and soothe various global issues such as war and famine. This world ends up at almost utopian levels.
All these butterfly effects springing out from Batman killing the men, women, robots and various other entities who threaten to harm the people of his world. Admittedly, the world does ultimately fall apart when Monarch invades the Multiverse but all the Justice League has to do is keep up their training and maybe look further afield than their own single planet and they would have remained safe.
Maybe killing the Joker isn’t exactly the best idea but the indomitable Justice League has other tricks up its collective sleeve. I can practically imagine the conversation now.
Batman: Joker broke out of Arkham and killed 50,000 people. Again. We need a different prison.
Superman: Sorry, the Phantom Zone holds Kryptonians.
Green Lantern: I handle space villains.
Wonder Woman: Women only.
Flash: I just froze Inertia because I hated his goggles.
Martian Manhunter: Not my planet, not my problem.
Aquaman: King of the fish-people. Emphasis on fish.
Atom: Shrinking down to atomic size is for science and running away.
Batman:… Seriously?
Even if they decide against a magical lobotomy (which didn’t turn out that well for them the first time) surely they could just ask for some help from their friends in the future? Everybody’s quite chummy with the 30th century and Wally West has his own friend in the 853rd century Justice League, the Justice Legion Alpha. If by the time that sentient computer viruses pose a danger to the public and the original Superman has ascended to god-like levels of power in the heart of the sun, civilization still has no cure for the Joker’s little problem then it’s probably a sign. He’s a bad egg. Even when the Spectre, supposedly the instrument of God’s vengeance, enters Joker’s mind, he finds that the maniac has no conscience. For a brief moment, the Spectre is able to show him what he has been doing all the years and the Joker willingly admits that he doesn’t deserve to live.
Green Lantern is authorized to use lethal force. Aquaman is the king of the seas. Martian Manhunter decided the fate of the remaining members of his race. Wonder Woman is a demi-god. Superman was the lone survivor of Krypton. Flash is a policeman. Time after time, they let the mass murderers loose onto the universe with only their moral code to hold them back when the law has clear punishment waiting. Batman tallied the numbers and created a paradise, one seen by several other heroes. Why not pull the trigger and let history take its course?