Batman and the Joker. Two of pop culture's most iconic heroes and villains, respectively, have duked it out for over 80 years at this point, and one question always comes about regarding these two characters: Why doesn't Batman simply kill the Joker?
A popular response is always Batman's no-killing rule, but many would argue that that's a weak comeback to keeping a monster loose in the world for so long. The other obvious answer is simply DC the company would never kill off one of their most popular villains, even less so after the Joker's popular first solo movie. But, is there another logical answer within the confines of the DC Universe as to why Batman doesn't kill Joker? Come with me to find the answer and to hopefully put this argument to bed once and for all!
Batman and Jokers' run-in with the Spectre
The Spectre was once a New York cop named Jim Corrigan during the 1930s but was one day caught and killed by mobsters he was pursuing. His story didn't end there, as stuck in Limbo, he felt that his death was unjust and that there was too much injustice in the world. Someone heard his cries, and that Someone was God, who offered Corrigan the power to serve Him and deliver justice to those who've suffered under evil's heel for too long. Corrigan agreed, and thus, imbued with great power, Corrigan became the Spectre, God's Wrath Personified on Earth.
During the 90s, the Spectre character was given his own series with writer John Ostrander and artist Tom Mandrake, with the goal of making a not popular character compelling for readers. To me, they accomplished this goal in spades, but what does the Spectre have to do with Batman not killing Joker?
Batman guest starred in issue number 51 of the 90's Spectre series, in his pursuit of the Joker, who made his way to New York City, hometown of the Spectre.
The Spectre and Batman manage to track down The Joker, but Spectre wants to deal with the Joker in his own fashion, against Batman's wishes. Spectre possesses The Joker to get a glimpse at what made the Joker tick, but Joker manages to not only trap the Corrigan side of the Spectre within himself, but Joker manages to take control of the Spectre's god-like powers.
With these new powers, the Joker uses them to bring his brand of terror and murder to the nth degree, killing cops and innocent people so fast and so brutally that Batman has no chance in hell to stop him. Inside the Joker's soul, Corrigan discovers that the Joker doesn't truly understand the difference between good and evil, which allows Corrigan to come up with an idea. Using his will, Corrigan regains control of the Spectre, but not before giving the Joker the understanding of what his heinous actions have done to the people he's hurt and killed, which makes the Joker shudder at his actions and fall into a catatonic state.
In the aftermath, Batman tells the Spectre that he doesn't believe in God and that the Spectre's methods for dulling out justice are anathema to him, but he did feel that The Joker never fully understood the consequences of his action even before Corrigan discovered the truth, and that's why he never killed The Joker in the past, because how can you kill someone who doesn't understand the difference between good and evil? And with that, Batman takes Joker back to Gotham, and the Spectre returns to his duty.
So, there you have the in-universe reason why Batman doesn't kill the Joker. After decades of stories and mayhem within those stories caused by Joker, is this answer satisfactory? I believe that it is a solid answer because this is my reasoning: Let's say a wolf kills a herd of sheep, and the shepherd hates the wolf for doing the deed, but can the shepherd really blame the wolf for doing something without really knowing or understanding the consequences of its actions? The Joker is a wild animal, in this sense, and while his actions are his own, he simply doesn't have the capacity to discern between right and wrong, which in Batman's mind allows the Joker to live to terrorize another day.