Imminent Death.

SLING BLADE SPOILER ALERT!
Though not explicitly stated at the beginning of the film as was the death of Lester Burnham in American Beauty, the death of Doyle in Billy Bob Thornton's Sling Blade is just as imminent. We are introduced to the character Karl, a mentally challenged man who murdered his mother and her lover with a Kaiser Blade (some people call it a Sling Blade) when he was a young boy. After many, many years of being locked up in a mental institution, he is released into the free world. Because of the abuse inflicted upon him by his father, Karl's issues with the free world arise when he meets Frank. Frank is a young boy whose father died when he was younger, leaving him and his mother to live life together. His mother sought out a new boyfriend, and unfortunately wound up in the hands of Doyle, an abusive drunkard who loves controlling people.
As Karl befriends young Frank, he learns of his troubles with Doyle. And as Frank learns of Karl's current situation (Frank is living in the back of a repair shop), a deep friendship forms between the two. Frank invites Karl to live with he and his mother, and this is when the audience knows that Karl will kill Doyle. The moment the audience actually meets Doyle, they see him for what he is: a man only concerned with himself, only with a family to make himself look better, seeking to control everything, and never relenting on degrading those around him. He is the abusive father figure in Karl's life once again.
Karl understands, even with his mental challenges, that killing is wrong. But why it is wrong remains elusive to him. The fact that he'd been locked up in a mental institution for forty years makes his understanding of the world very limited. And indeed, after all is said and done and Karl winds back up in the institution, he claims that the world was just "too big" for him.