Who is the Protagonist of this movie?

there isn't one. unfortunately. i know that i am in the minority here, but while i enjoyed the film, and while i also understand that it is supposed to be a dark comedy, the lead up to the ending and the actual ending disappointed me. until a certain moment douglas' character was simply a dude that was fed up with getting crapped upon and completely lost it. to me it was a copout in the final act to reveal him to be an unstable character to begin with and end it the way it it. very safe ending as well and it makes the overall buildup a bit meaningless. i would not insist on some giant twist or him eventually getting away with it. an different third act and an open ending would have been powerful enough, but ending with "oh, well. he was a psycho/douchebag anyways", is just something cowardly to come up with.

It depends on the situation.

In some cases, Bill himself serves as the protagonist. In particular, the situation in the park when the bangers come up and start hassling him, and he fends them off. Then a bit later, when they do a drive-by on the street and he walks over and shoots one in the leg, vigilante style. It's hard to say he's really acting like a "bad guy" in those situations.

When the beggar comes up to him and starts shaking him down for money, and Bill begins tossing it back at him, and leaves him with the empty suitcase, that's probably endearing to the audience.

The golf course scene, where he scares the crap out of the moron acting like he owns the world, is also sort of a moment that the audience can cheer for. But it takes a bit of a sour turn when the man starts dying, and Bill begins to mock him, and relishes in his impending death.

The surplus store, is another such moment, where, although Nick is a bastard, it's hard to argue that he deserves to die. Yet Bill ends up murdering him. Or the Whammy Burger, shooting up the place and scaring the hell out of the patrons just because they wouldn't serve him breakfast, regardless of whether they had the food or not. At these times, he's very much gone from the world of the protagonist and has become a menace to general society.

Terrorizing his ex-wife is obviously him playing the part of the villain. Although we can empathize with him just wanting to see his daughter, it's understandable that he's upset that he can't be there for her birthday, but it's still an issue that went through the courts, and him taking matters into his own hand isn't exactly on the up-and-up.

Throughout the movie, however, Prendergast and Sandra are just trying to solve a spate of events and crimes, in pursuit of justice and peace. I'd have to say that Prendergast mainly, and Sandra secondarily, are the main protagonists of the film.

While Bill may dip in and out of protagonist-esque behavior, in the end, he's the antagonist driving the entire affair.

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Kevin... still not wearing any pants, I see...

I would say both D-fens and Prendergast are protagonists. But by the end of the movie, Prendergast is the protagonist and D-fens has morphed into the villain.

Duvall, of course.

I think Bill's life made sense up until his wife divorcing him and then losing his job at the defense plant.

We all need lives that make sense, that seem fair and coherent.

Obviously Bill loves and needs order in his life, hence the perfectly kept and extremely tidy bedroom at his mother's house. The more one can not stand disorder in one's life, the greater the potential for violence when things fall apart.

Robert Duvall's character lets him know there is not necessarily fairness in one's life, giving the example of his young daughter's death. Sometimes there is nothing but extreme unfairness.

Both D-fens and Prendergast.

The Michael Douglas character.

Both Duvall and Douglass characters are protagonists. I’m rooting for both, even if I see Bill is sick in the head. While I don’t condone his actions, I understand them. Being an adult is frustrating, especially when everything falls apart, and I’ve wished I could react in the ways he did. And I’m sure Duvall wished he could too.

Bill and Duvall are character foils. They have many similarities, but their differences are supposed to highlight their differing qualities.