Yes, Harley Quinn season 3, episode 6, "Joker: The Killing Vote" really did just fix the worst thing that the Joker has ever done in the comics. Many DC Comics readers will recognize the title of the episode as a reference to Alan Moore and Brian Bolland's "The Killing Joke," the controversial one-shot graphic novel from 1988. However, though the Harley Quinn episode where Joker becomes mayor of Gotham echoes "The Killing Joke," its plot is actually based on the 2017 limited comic book series "Batman: White Knight." By satirizing these two DC Comics storylines, Harley Quinn succeeds at reimagining Joker's worst crime for modern audiences.
In "The Killing Joke," Joker invades the Gordon home to shoot and paralyze Barbara and kidnap Commissioner Jim Gordon. The Joker physically and psychologically tortures Gordon inside an abandoned amusement park - an attempt to drive him insane and prove that even the Police Commissioner is no different from the Joker. It is then revealed that Joker not only shot Barbara, he also took off her clothes with the intention of breaking the Commissioner by showing him photos of what he had done. Today, the treatment of Barbara Gordon in "The Killing Joke" continues to be a divisive topic in the DC fandom, with some calling it unnecessarily exploitative, and others deeming it crucial for both Joker and Batgirl's respective character arcs.
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With Harley Quinn adapting "Batman: White Knight" and "The Killing Joke" for "Joker: The Killing Vote," an entirely different version of events is imagined for both Barbara and Joker. Rather than the quick removal of Barbara Gordon as Batgirl through horrific means - before her later return as Oracle - Barbara in "Joker: The Killing Vote" becomes the sole voice of reason in her father Jim's mayoral campaign. Instead of Joker committing his most heinous crime, in Harley Quinn Joker is instead here a version of the villain-turned-politician from "Batman: White Knight." Through Jim Gordon's help, Joker even succeeds at preventing Two-Face from doing to him what he tried to do to Jim in "The Killing Joke." Harley Quinn takes the familiar trappings of an abandoned amusement park and completely subverts the source material by delivering totally different arcs and endings for both Joker and Barbara.
Alan Moore's "The Killing Joke," at its core, is really about Batman's no-killing rule. Jim and Batman both believed that the right thing to do, despite what Joker did to Barbara, was still to capture Joker alive and put him through the justice system. In terms of exploring whether keeping Joker alive is justified, some argue that it was necessary to see Joker performing his worst crime - though there's certainly merit to the opinion that what happened to Batgirl was completely exploitative. In "Joker: The Killing Vote," Harley Quinn gives Barbara back her agency in a way that pays proper homage to both comic books that inspired the episode.
The Harley Quinn adaptation of "Batman: White Knight" lays the perfect groundwork for redeeming Barbara's "The Killing Joke" fate. Realistically, it has done so since its introduction of Barbara Gordon, who serves as one of the few proficient and relatively well-adjusted figures in the animated series. In a ridiculous world where suburban Full House dad Joker becomes mayor of Gotham on a progressive platform, where Jim Gordon is just an incompetent stooge for Two-Face, and Batman is nowhere to be found, Gotham's fate could be in Barbara's hands.
The episode not only plays with the imagery of the amusement park where Batman and Joker have their "Killing Joke" showdown, but has the narrative tied to Barbara trying to convince her father to stand for something meaningful, putting her in a position of wider change. This is a much better fate for Batgirl, rather than simply removing a key female character from the story as "The Killing Joke" attempted to do. Through the Harley Quinn show and its weird and wonderful sense of humor, the series is able to imagine an alternate reality where anything is possible - including fixing the matter of the Joker's worst crime and most renowned controversy via playing with the tropes and settings that come with it.