Arthur Fleck was an amateur stand up comedian and a professional clown. He lost his clown job after being framed by a co-worker and was repeatedly beaten up by various people after unsuccessfully trying to resurrect his career.
Struggling with mental illness and a laughing disorder, he lost access to his medication and therapist, causing him to succumb to madness and kill his deceitful mother.
His abusive treatment by the people of Gotham City led him to become the vengeful, sociopathic anarchist known as The Joker.