He is not political, he is a clown, comedian. He just wants to make people laugh.
People don't find his jokes funny. Arthur doesn't think people's jokes are funny.
Arthur Fleck gets treated badly by almost everyone.
Kids on the street, a co-worker, employer, Thomas Wayne, his hero, his mom: she had abused him in the past due to her illness.
He kills the three assholes in the subway. He kills his mom. He kills his co-worker. He accepts the name his hero gives him, Joker and kills his hero.