Matt Fowley is a caring and dedicated father. He has always taken his role as protector of his wife and children very seriously. When Richard Strout takes one of his children away from him, Matt is unable to accept the loss of his son, leading him to committ murder. After his son's murder, Matt feels the anguish of his wife and thinks it's his responsibility to try to alleviate it in some way. As he and Richard drive to the woods, Matt looks at Richard's fists, knowing Richard had used them to hurt Frank. Matt also thinks of his son's last moments before death, having no idea of what was about to happen to him. These thoughts encourage Matt to continue his murder of Richard.
Is Matt Fowley insane? I'm sure a defense attorney could make a good case for it, citing Matt's overwhelming grief at the sudden death of his son. However, Matt carefully plans his cold-blooded killing of Richard. He and Willis premeditate the crime, and Matt's wife also knows what Matt is going to do. Neither Willis nor Ruth, Matt's wife, feels what Matt is doing is wrong. More importantly, Matt doesn't feel he's doing anything wrong; he feels he's getting justice for his son. Matt was unable to protect his son from Richard and that guilt won't go away if Matt allows Richard to live.