Who is the Protagonist of this movie?

Prendergast. There are moments when D-fens feels like the protagonist but those moments of sympathy never last long to me. He has experienced injustice and frustration but at the same time his worlds view is quite distorted and he is clearly capable of some serious violence. Difficult to identify with him.

There are two major character Arcs. Bill who is becoming unhinged and Pendergast who is becoming involved in policing.

I say Bill is the main protagonist since his story is the focus of every character's action in the film. The involvement of Pendergast is to keep Bill from going any further. By the End, Bill doesn't recognize how far he's gone.

I feel like some people are confusing the word protagonist with the word hero. They are very different terms.

D-Fens is the hero, you're just not supposed to say so out loud.

Both Pendergast and D-fens are protagonists. Pendergast is a hero. D-fens is an anti-hero.