Mary Meadows (Katie Holmes) vigilantism stems from the senseless death of her mother Mrs. Meadows (Jean Smart) who was killed in front of her as a child.
Mike (James Badge Dale), the local Sheriff she is engaged to (and pregnant with his child) confronts Mary and tells her he knows she's responsible for the string of killings but is willing to bury it if she'll stop. Mary promises to try and they proceed towards their wedding. On her wedding day, Mary and one of her students, Heather, are kidnapped by Skylar (Callan Mullvey) a released pedophile who Mary threatened with violence before. Mary is able to help Heather escape and before Skylar can kill Mary, Mike shows up thanks to Heather's help and shoots him dead. Since Skylar had Mary's gun they can blame him for the killings.
The film ends one year later, with Mike, Mary and their baby daughter in a big new house. Mary says she has to leave for a bit, and given we are shown a close up of her purse and Mike warning her to "be careful" implies that Mary hasn't completely given up being a vigilante killer (and that Mike has grown to accept it).