Sling Blade (1996)

Written, directed, and starring Billy Bob Thornton as an intellectually disabled man released from a psychiatric hospital after killing his mother and her lover

Karl Childers killed his mother and her teenage lover when he was 12-years-old, leaving him incarcerated in a psychiatric hospital until his release decades later. Upon his release, he meets 12-year-old Frank, and his mother Linda, who invites him to live in her garage with her boyfriend Doyle. But tensions rise when Karl discovers that Doyle is abusive to Linda, and threatens to kill her and Frank.