Why does Willis help Matt take revenge?
This becomes one of the most interesting elements of the narrative. There is little in Willis' character to indicate that he would be privy to being such an agent of violence. However, there are elements to his character which operate upon him, causing a pivot to being a collaborator in a heinous crime. Willis is a part of the town gossip machine. When Matt comes to him in an emotionally broken state, Willis does little to assuage Matt. Rather, he helps to fuel his condition with talk of unsolved murders in the town, the easy access to guns, and revenge.
In a way, being privy to the town gossip has enabled Willis to easily enter into a domain that previously would be unthinkable. Being able to live out the elements of gossip through Matt's grief and banking on it to fulfill these tendencies is where Willis is able to help Matt. When Matt begins to engage in some level of doubt about the implications of what he is to do, Willis is steady and resolute in following the plan of premeditated murder. It becomes clear that Willis is helping out Matt in understanding his pain. Yet, a part of the reason he is able to do so is through his role in the town's gossip machine in which information and innuendo are taken to mean more than what they might. As opposed to being a force of moral clarity, Willis is really no different than any of the other main characters who operate in a realm where moral ambiguity reigns supreme.