Southern cuisine is defined by being simple but delicious (and artery-clogging). Karl's favorite food falls into the simple category for sure.
What is it? French-fried potaters, as Karl is known to say it. It's almost as fun to hear him say it as it is to eat them. Who doesn't like French fries? Okay, most people don't have them as their entire meal, the way Karl does, but at least he's getting a full serving of vegetable. Yes, we said vegetable. It's a lot, but it's a lot of one vegetable.
And it might be the only vegetable Karl eats. His other favorite food is biscuits with mustard. We haven't tried that delicacy yet.
Karl is easy to please, but the people who care about him are comfortable letting him have the foods he likes. They don't try and force him to try new things or to diversify his diet. Bill Cox brings Karl a big ol' basket of fries for lunch, and Linda makes Karl biscuits for breakfast, and nothing else.
Karl appreciates it.
KARL: You been real good to me too. It ain't everybody that'd make biscuits in the middle of the night. You and that boy's given me a good feeling.
After killing Doyle, Karl calmly sits at the kitchen table and eats a biscuit. For Karl, it's the simple pleasures that are the important ones. These yummy bites are his final meal as a free man. He enjoys his favorite food one last time before returning to the mental hospital—that biscuit-less land of padded walls.