Independent film from 1996, directed, written by, and starring Billy Bob Thornton as Karl Childers, as a mentally handicapped man who is released from a mental hospital decades after he killed his mother and her lover. He moves back to his hometown, where he meets some new friends and gets into more trouble. (The actual filming was done in Benton, Arkansas; a small town about 20 miles south of Little Rock.)
The film also stars Dwight Yoakam, John Ritter, Lucas Black, J. T. Walsh, and Robert Duvall. It was preceded by a short film Thornton created called "Some Folks Call it a Sling Blade". Thornton won an Academy Award for his screenplay and was nominated for his acting -- he convincingly transformed himself from a short, balding guy into a stoic, gravel-voiced, hulking man-child with a penchant for engines, religion, and violence.
Also noteworthy were two actors playing strongly against the audience's expectations. John Ritter, best known as a comic actor and physical comedian, played a gay discount store manager as a fully realized dramatic role. And Dwight Yoakam, best known for his laid-back, chill performances as a country music performer, plays Doyle as a terrifying, abusive hatemonger.
This is one of the few movies I know that appeals to both film snobs and people who hate film snobs -- everyone seems to love it...