The film, Joker, is in essence a tragedy acted by a comedian. One of the central themes of the film is that laughter and happiness are not necessarily interchangeable. That is to say, it’s wrong to conclude that those who laugh a lot and loudly are undoubtedly happy people.
Arthur was an innocent victim of the unabated cruelty and inequity of his society. He was suppressing all the oppression imposed on him for an extended period of time until he got a chance to take revenge. This action, which is not supported anyhow, made him feel that he was no longer a victim.
It is clear that Arthur was not a vicious or wicked person as he did not do any harm to the dwarf or midget in the film, and in fact, tried to protect him. Psychologically speaking, people who are traumatized or victimized at an early age like Arthur could probably resort to violence or crime when they grow up.