The ending to Sling Blade is shocking in how not shocking it is. Unless you slept through the first half hour, you're expecting Karl to kill someone over the course of the movie. Along the way, in fact, it's hard to decide whether we hope Karl will kill Doyle, or whether we hope that he won't.
If you hoped he wouldn't, and that he would instead remain with Frank and Linda as the weird garage-dwelling member of their family, you must have been disappointed when Karl plunged the freshly sharpened lawnmower blade into Doyle's skull.
Sling Blade isn't a movie with M. Night Shyamalan-style plot twists. Karl has been alive this whole time, and the plants aren't trying to kill people. No, this is a simple-but-complex film about Karl's inevitable journey to killing Doyle. But was it really inevitable? When Karl goes to Vaughan before killing Doyle, Vaughan must know what Karl is about to do, right? But Vaughan doesn't try to stop him. Why?
Whatever the whys, Karl ends up back in the mental institution. This ending brings us full circle, back to where we began. We're going in circles, which reminds us of the definition of insanity we've read online when attempting to psychoanalyze ourselves: it's doing the same thing over and over again but expecting different results. Except that Karl didn't expect different results: he wanted to go back to the hospital. Does that make him sane?
Returning to the mental hospital puts Karl alongside his foil, Charles Bushman, once again. This time, the contrast between them is much clearer than it was at the film's beginning. Charles kills people selfishly, because he wants to possess them in a sick way. Karl believes that the murder he committed was selfless, done to save Frank's life.
This conversation between these men tells us the real reason Karl wanted to return to the hospital's monochrome world.
CHARLES: What was it like out there in the world?
KARL: It was too big.
Karl prefers a world where most big decisions are made for him and the only choices he must make for himself are simple ones. When Charles leaves, dragging his chair away, Karl remains staring out the window. Maybe he's wondering what life is like for Frank and Linda left in the world without him.