KARL: I like your garage. I never would hurt you or that boy. I'd lay my hand on the Bible and say the same thing.
Karl believes that his word will get some extra oomph if he lays his hand on a Bible. That shows us that he had a religious upbringing, and that he takes it somewhat seriously.
KARL: I wasn't but six or eight. I reckon I didn't know what to do. I didn't know how to care for no baby. My mother and father didn't want him. They learned me to do what they told me to do. These days I figure it probably best if we just give him right back to the good Lord right off the bat anyhow.
Karl gives the premature baby a merciful death because he believes the child will return to the Lord. Frank says he would have tried to keep the baby alive. Is that because Frank isn't religious? Or are there other reasons?
FRANK: I didn't mean to say nothing bad about you. You're good. You don't mean no harm. You ever think about killing yourself on purpose, like Daddy did?
KARL: I studied about it some. Bible says you ought not to. Says you do that, you go off to Hades. Mmhmm. Some folks call it hell. I call it Hades.
FRANK: Bible says the same thing about killing others too.
KARL: Yes, sir. I reckon it does.
This exchange is the beginning of Karl's questioning of his own religion. He thinks Frank's dad is a good man, and he can't imagine that he would go to Hades for killing himself. We're not sure if Karl thinks he is going to hell himself, but he doesn't feel any guilt for the murders he committed. He is starting to think that the Bible isn't 100% in line with his own personal beliefs.
KARL: I learned to read some. I read the Bible quite a bit. I can't understand all of it, but I reckon I understand a good deal of it. Them stories you and Mama told me, they ain't in there. You ought not done that to your boy. I studied on killing you. I studied about it quite a bit. But I don't reckon there's no need for it if all you're gonna do is sit there in that chair. You'll be dead soon enough. And the world'll be shut of you. You ought not kill my little brother. He ought of had a chance to growed up. He would have had fun sometimes.
Karl appears to have been raised in a fundamentalist household. We're not sure what stories they told him that weren't in the Bible, but it appears they thought that by keeping Karl from reading, he wouldn't learn to read the Bible and question them. Knowledge is power.
KARL: I wanna be baptized.
Even with his mild religious confusion, Karl wants to be baptized. But why? What does he still believe in? To us, it seems like Karl is getting baptized as a safeguard before killing Doyle. Maybe if he's baptized before he does it, his soul will be safe.
KARL: I don't reckon that you have to go with women to be a good daddy to a boy. You been real square dealing with me. Bible says two men ought not lay together. But I bet you the good Lord wouldn't send nobody like you to Hades.
Karl is a kind of Jesus figure, in a way. He considers himself a savior, so it's not surprising that he thinks he can revise the Bible to fit his newfound belief system.