Great film, highly recommended but make sure to give yourself time to recuperate. I was motivated to write a review because of how many articles were saying the film drew a dangerous and inaccurate correlation between mental illness and violence. I did not feel this was what the film portrayed at all.
***Spoilers***
Arthur did not murder because he was mentally ill. He murdered because he was repeatedly beat up and pushed into a hopeless corner in society. The initial murder on the train was accidental and he feared for his life. This initial murder flipped a switch in him as it would in many people. You committed a taboo and you got away with it. Or so you thought. He murders her mom after discovering the life long lie/betrayal by her. Someone who he trusted with his life and who was closest to him. Then he was betrayed by the person he looked up to and commits his final murder. Each murder becomes easier than the last.
Jordan Peterson once said that he witnessed people overcoming all kinds struggles in life. Whether it was critical illness in themselves or their loved ones, bankruptcies, amputation, lost of senses, anything you can think of. Except for one thing. Betrayal, especially betrayal from people you thought you could trust. Betrayal from people who you trusted with your own life and livelihood. I think this movie portrayed this well. Mental illness was just the cherry on top.