The book explores Moore's assertion that, psychologically, "Batman and the Joker are mirror images of each other"[6] by delving into the relationship between the two. The story itself shows how the Joker and Batman came to terms with their respective life-altering tragedies, which both eventually lead to their present lives and confrontation. Critic Geoff Klock further explained that "both Batman and the Joker are creations of a random and tragic 'one bad day'. Batman spends his life forging meaning from the random tragedy, whereas the Joker reflects the absurdity of life, and all its random injustice".[7]
The torments that the Joker puts Commissioner Gordon through are meant to serve as "proof that there is something buried deep within each lunatic, a nugget of insanity, that is simply waiting for the right moment to spring forth".[8] Unlike the Joker, however, Gordon emerges from his ordeal with his sanity and moral code intact. The story is also famous for returning Batman to his darker roots, which changed how the Modern Age of Comics perceived the character.[9] The Killing Joke delves deep into the character's psychology, framing him as being just as insane as the Joker, and postulating that he and the Joker perceive the world according to differing points of view, with the Joker viewing the world as one big joke.[10]
The final panels of the book have been the subject of interpretation and debate.[11] According to one view, Batman and the Joker, who have been fighting for years, end all of their disputes by having a good laugh about it all.[12] Another interpretation, popularized by comic writer Grant Morrison, is that Batman kills the Joker at the end of the story, hence the title "The Killing Joke". Grant Morrison acknowledges the ambiguous nature of the story's ending, which they believe is a part of its enduring appeal, with the original script providing no further information.[13]
The Joker serves as an unreliable narrator. He admits to his own uncertainty, as he has disparate memories of the single event ("Sometimes I remember it one way, sometimes another...If I'm going to have a past, I prefer it to be multiple choice!"), accentuating the comic's depiction of "a world unraveling toward relentless urban violence and moral nihilism".[14]
