Zach Snyder's Batman v Superman offered a controversial dark knight that was truly beginning to fall into the darkness he claims to fight (the extended addition makes that even more clear than the theatrical version). The Killing Joke (which is based on the 1988 story arc from Alan Moore and Brain Bolland) builds on this trajectory in a move that might rightly be called Breaking Bat.You can find summaries of the plot elsewhere; I prefer to focus on the worldview within the movie.
A flashback to the Joker's early life reveals the first meaning of the title, The Killing Joke. He was a struggling comedian looking for a joke that ‘killed,’ the one that got the audience rolling with laughter. He was desperate to support his pregnant wife and do something meaningful in the world. When some criminals approach him about helping them with a heist, he reluctantly agrees. One big score is all he needs to keep his family – and his dream of making people laugh - afloat. It falls apart, of course. On the day of the robbery, he receive that news that his wife and unborn baby have died. The mobsters won’t let him off the hook. That night, he falls into a vat of acid while running away from Batman and emerges as the Joker.
That’s where we find the second meaning to the title. Life is the killing joke. Life is “a tale told by an idiot, full of sound and fury, signifying nothing.” The Joker is the comic book world’s premier nihilist. He just wants to watch the world burn. If that’s not possible, he will do everything he can to burn away the goodness and sanity of those around him.
He doesn’t think it will be that hard. He believes that everyone is just one bad day away from embracing the insanity that has become his closest friend. One bad day where everything you love is ripped away. One bad day where all hope is lost. He seems truly convinces that even Batman or Commissioner Gordon would join him on the other side of reality if what happened to him happened to them.
This provides the plot to the story. The Joker does his best to force Gordon and Batman into that one bad day to prove his theory correct. If he is successful – if he can take the two best people he knows and break them not just bad but insane - he will be vindicated.