…how do you get this from The Killing Joke? It's Joker's origin story, straight up. It's the contrast of how Batman's pathology turns him driven to enforce order through justice, where Joker's pushes him to enact chaos because the universe is inherently unjust.

like boredfrom pointed out just below my previous post, some people read this panel more strongly than others
e: the strong reading is that joker’s been fudging/omitting information, if not just completely making shit up. Some people like that because they like a joker who’s this manipulative and because it salvages the mystique around the character. Its also completely compatible that the details of the ‘origin story’ are bunk, but that they genuinely illustrate the themes that you mention above.