Given the question: Build a movie plot around this: Why did he killed his mother? he discovered that his mother was a willing participant in the affair.
The answer is:
Karl Childers (Billy Bob Thornton) is an intellectually disabled Arkansas man who has been in the custody of the state mental hospital since the age of 12 for having killed his mother and her lover. Although thoroughly institutionalized, Karl is deemed fit to be released into the outside world. Prior to his release, he is interviewed by a local college newspaper reporter, to whom he recounts committing the murders with a Kaiser blade, saying, "Some folks call it a sling blade. I call it a kaiser blade." Karl says he thought the man was raping his mother. When he discovered that his mother was a willing participant in the affair, he killed her also.Thanks to the doctor in charge of his institutionalization (James Hampton), Karl lands a job at a repair shop in the small town where he was born and raised. He befriends 12-year-old Frank Wheatley (Lucas Black) and shares some of the details of his past, including the killings. Frank reveals that his father was killed - hit by a train - leaving him and his mother on their own. He later admits that he lied, and that his father committed suicide.Frank introduces Karl to his mother, Linda (Natalie Canerday), as well as her gay friend, Vaughan Cunningham (John Ritter), the manager of the dollar store where she is employed. Despite Vaughan's concerns about Karl's history in the mental hospital, Linda allows him to move into her garage, which angers Linda's abusive alcoholic boyfriend, Doyle Hargraves (Dwight Yoakam). Karl bonds with Linda, who makes him biscuits to eat, and also with Vaughan, who tells Karl that a gay man and a mentally challenged man face similar obstacles of intolerance and ridicule in small-town America.Karl quickly becomes a father figure to Frank, who misses his father and despises Doyle. Karl is haunted by the task given to him by his parents when he was six or eight years old to dispose of his premature, unwanted, newborn brother. He visits his father (Robert Duvall), who has become a mentally unbalanced hermit living in the dilapidated home...