If Sling Blade were a novel, it would be written in first person from Karl's perspective, and Billy Bob Thornton's typewriter would have a broken "m" key from all of Karl's mumbling.
The tight perspective on Karl means that Thornton himself is in every scene. We never see what happens elsewhere, meaning that we only know what Karl sees and what he is told. That makes us wonder, are Frank and Vaughan reliable sources of information?
Karl is very simple. He isn't going to analyze what he is told; instead he'll take it at face value. Vaughan tells Karl that Doyle is a "monster." Frank says he wants to kill Doyle. Karl takes all this literally.
Doyle is not a nice man, that's for sure. But is he violent? He threatens Linda's life verbally, but he never attacks her. After she pushes him, he pushes her back. Then, while Frank throws things at him, Doyle lies on the floor and takes it. So is he dangerous? It's not 100% clear.
Another question brought up by the narrative style is whether or not Vaughan and Frank are taking advantage of Karl. Consciously or unconsciously, the two plant the seed of murder in Karl's fertile mind. With the perspective so tight on Karl, we don't get to see how these two react to Doyle's murder. Do you think they're relieved?