Arthur Fleck is not the protagonist of the film, neither is Harley Quinn
The real protagonist of Joker: Folie a Deux is the society — a thread
Joker: Folie a Deux has a complicated storyline where both, Joker and Harley, seems to act like someone unknown above them is trying to affect their actions.
The society (in the film represented by the people of Gotham) paints Arthur Fleck as the pioneer of the citizen anarchism movement.