Because I saw some discussion earlier that this movie could likely be a figment of Arthur's imagination or much like how he fantasized about being in a relationship with his neighbor, I feel that is factually wrong. The whole plot of the movie, the conclusion of his character and his descent into madness and ultimately becoming the Joker is exactly because of the events that led up to it. His mental illness of inappropriate laughter was warped into a mental justification of his actions. He ended up finding a stage for himself with the ultimate joke, the ultimate stand up routine of life. He eventually turned his inappropriate laughter into his normal behaviour, finding killing and torture genuinely funny, chaos endearing, and joy in the destruction of his world. He says as much to Murray, comedy is subjective, and the ending in Arkham solidifies it, in his subjective view the murder of his doctor was a punchline in his eventual escape. This is also why I feel that saying this movie has no plot or meaning, just a slow burn and all style but no substamce is disheartening. No one really payed attention to the metaphorical, and how everything in the movie is a deconstruction and interpretation of every aspect in the Joker's storied history. From the name Arthur Fleck to the iconic laughter and look, this Joker's creation is a culmination of a perfect storm.