Vaughan is a gay man in Arkansas, which means he might as well be a Muppet at an Insane Clown Posse concert.
The film is set in a vague "present day," which would put it in the mid-90s. But we'd forgive you if you thought it was set in the 1950s. Time moves slowly in rural Arkansas, so even in the mid-90s, no one has computers, cellular phones, or any understanding or acceptance of homosexuality.
As a result, Vaughan feels obligated to live a closeted life. However, aside from a few rude remarks, Doyle appears to tolerate Vaughan. He never threatens to expose Vaughan, nor is he ever violent against him because he is gay. That's one more detail that keeps Doyle from being a total villain.
But still, so by virtue of being gay, Vaughan is disqualified from being a father figure to Frank, even though he would be a better one than Karl or Doyle. According to Linda:
LINDA: You know [Frank's] always after a father figure. […] I don't think he sees you as a "guy" guy.
Remarks like these not only hurt Vaughan's feelings, but they make him feel like less of a man, or less of a person entirely.
Half the Man He Used to Be
Vaughan believes he has a kinship with Karl, who is also viewed as less of a person by the majority of the town because of his mental illness. Even today, a lot of people think homosexuality is a mental illness.
So, feeling a kinship with Karl, Vaughan opens up to him.
VAUGHAN: You see, you and I are a lot alike, as strange as that may seem. I don't mean physically or even mentally, really, but emotionally. Actually, the hand that we've been dealt in life…We're different. People see us as being different, anyway. You're…Well, you've got your affliction or whatever and I…Well, mine's not as easy to see. I'm just going to say it. I'm gay.
From a certain perspective, it might be offensive that Vaughan is equating himself with a person who is mentally ill, as if being gay were a mental illness. However, where Vaughan lives, homosexuality might as well be. Vaughan feels a need to keep his sexuality a secret, so in a way, he envies Karl's ability to limp around eating all the French-fried potaters he wants in broad daylight.
Vaughan feels like living a closeted life is a sacrifice he makes for Linda and Frank. He says as much to Karl:
VAUGHAN: I've wanted to leave many times, but, because I love Linda, Frank, and a certain other person…I…They've kept me from leaving...
For Vaughan, staying local is like living in a mental institution when all he wants to do is get the heck out of town. But neither Linda nor Frank has asked him to stay, and don't you think they would rather Vaughan leave if he were happier elsewhere? Vaughan comes across as having a bit of a martyr complex, maybe.
Vaughan isn't willing to go full martyr and totally sacrifice himself for Linda and Frank, though. Instead, he relies on Karl to make the sacrifice for him.
Yes, we are suggesting that Vaughan manipulates Karl into killing Doyle so that he doesn't have to. Consider the following. When Karl tells Vaughan to keep Linda and Frank away from their house, Vaughan must know that Karl is about to kill Doyle, right? If Vaughan knew that, and didn't stop it, then what are the implications of his silence? What would Linda think if she knew? And by allowing Karl to do it, is Vaughan letting Karl commit a crime that he knows he could never commit himself?
We know that this is one of the more cynical ways to view the movie, but we have to go there. After all, if Sling Blade teaches us anything, it's that people who feel trapped will take desperate measures to get out.