Marsha Dwiggins, the college student doing a report on Karl, is nothing more than a plot device to set up our story. She's more of a character in the short film the movie is based on, fleshed out there by Molly Ringwald. But here, all she does is ask Karl a key question.
MARSHA DWIGGINS: Will you ever kill anybody again, Karl?
KARL: I don't reckon I got no reason to kill nobody. Mm.
Karl's answer is telling in its honesty. Karl will spend the rest of the rest of the movie in peace…until Doyle gives him a reason to kill.