The Killing Joke/The Dark Knight Returns
If there are two comic book influences on Joker, it’s The Killing Joke and The Dark Knight Returns, which also previously inspired the Dark Knight trilogy.
The Killing Joke reveals that Joker was once a struggling stand-up comic who was living a poverty-stricken existence, which is a concept that Joker runs with, too. TKJ, however, has it taking just “one bad day” to turn this innocent man into the Joker while the movie paints it as the accumulation of an entire lifetime of bad days that transforms him. That said, Arthur does say he’s “had a bad day” at one point, calling back to Alan Moore’s graphic novel.
As for The Dark Knight Returns, Frank Miller’s seminal comic features a scene in which an apparently benign Joker appears on a talk show before ultimately revealing his sinister plans to kill the entire studio audience. Fleck doesn’t go that far, but the way his initially harmless act devolves into killing Murray definitely recalls that moment.