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.

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.