Who is the the protagonist in "Killings," and what are the two opposing forces that are pulling at him?

The protagonist in Dubus's "Killings" is Matt Fowler. He is the father of Frank who is killed by Richard Strout; and, Richard kills him because he was dating his estranged wife Mary Ann. The two opposing forces facing Matt Fowler are his and his wife's grief for the loss of their son and his sense of justice. First, Matt and his wife's grief for their murdered son is antagonized by the fact that the murderer is out on bail and walking around town as if he were free and clear of any responsibility. He even gets a job at a bar in the next town over; but Ruth, Matt's wife, doesn't feel comfortable even going to the store because she's afraid to run into the man who killed her son. The author describes Matt's grief as follows:

". . . and then he lost Frank in a way no father expected to lose his son, and he felt that all the fears he had borne while they were growing up, and all the grief he had been afraid of, had backed up like a huge wave and struck him on the beach and swept him out to sea."

Matt and Ruth's daily lives are filled with grief and pain for the loss of their son; and, Matt wants to relieve that grief no matter what it takes.

Then, when Matt articulates his concerns about his grieving wife to his friend, Willis helps to tug at Matt's sense of justice by reminding him of a woman who shot her husband and got away with it. Willis bets that Richard will only get five years in prison for Frank's murder. Not only is that an unsatisfactory number of years to serve for murder, if Ruth can't handle seeing Richard in town now, what would it be like in five years after he's served his sentence and allowed to roam free? In addition to the injustice discussed by the men about the justice system, Willis says that he will help Matt plan a way to take care of Richard.  

Other thoughts that trigger Matt's sense of justice center around the fact that Richard shot Frank in front of his two young son's, Mary Ann seemed painfully troubled while dating Frank, and so many people's lives have been hurt because of Richard. And to Matt, no one will receive justice for all of the pain that he has inflicted on them.