These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
He Really Can Act: Billy Bob Thornton is so known for playing Jerks and Sleazes that seeing Him as the gentle but unstable Karl is something of a shock.
Also, country singer Dwight Yoakam is surprisingly damn frightening as Doyle.
When promoting the film on Late Night With Conan O Brien, Yoakam shared an amusing anecdote about how he knew he had done a good job when after the premiere, his own mother looked him dead in the eyes and said " I'm glad he killed you!".
Hollywood Homely: Billy Bob Thornton was told by Billy Wilder that he was too ugly to be an actor and should write a screenplay to exploit his features
Moral Event Horizon: Doyle's may be when he flings poor Terrence in his wheelchair against the screen door in the act of kicking his band out.
Frank Childers's was carried out long before the events of the movie, as related by Karl: he expected Karl to dispose of his premature little brother as though he were trash; the follow-through to this unseen event is shown in the Heartwarming Moment above.
Some folks call me Shazam: Because of the film, "Sling Blade" is used by some as a derogatory term for the mentally challenged.
Tear Jerker: Karl relating to Frankie how his father made him dispose of his prematurely-born little brother in a trash barrel, when he was "six or eight"; even then it didn't seem right, so Karl took the tiny being and gave him a decent burial in the backyard.
To say Karl had a crappy life is one heck of an understatement. He was forced to live his childhood in a shed with a dirt floor, sleeping in a hole in the ground. He was bullied constantly by all the children his own age and treated like crap by everyone including his own parents right up until he killed his mother and her lover.