Here are a few words we'd use to describe Sling Blade: understated, quiet, brooding, and a little salty, like a basket of French-fried potaters. Haven't you ever wondered what those potatoes are thinking? They do have eyes, you know. They're watching you.
When we're not thinking of sentient, man-eating potatoes, we're thinking of a few words we wouldn't use to describe Sling Blade: sensational, gratuitous, explicit, sexual. There is no sex, although there is some risqué dialogue. And all violence occurs off-screen.
So why is it rated R?
The answer is Doyle. Doyle contributes "22 f-cks, 13 -sses, 12 sh-ts, 7 sons of b-tches, 3 t-ts, 3 pr-cks, 2 p-ssies, and 1 c-cksucker" to the script (source).
Heavens to Betsy.
Karl is like a human version of the MPAA. He's awfully concerned about language, telling young Frank not to swear because he is a child. But Frank most likely gets the bad language from Doyle. Maybe that's why Karl kills Doyle—to eliminate a total potty mouth from the world.