In The Grapes of Wrath, why does Uncle John have guilt that is driving himself crazy?

Uncle John is haunted by the death of his young wife many
years before. They had been married only four months. She had been pregnant. When she
complained of pain in her stomach, John dismissed it, saying she had eaten too much. She
died the next afternoon. "[S]omepin jus' bust in her. Ap-appendick or somepin," Tom
says. Uncle John's guilt over her death consumed him. Tom explains it in Chapter 8 of
the novel:

.
. . Uncle John, he's always been a easy-goin' fella, an' he takes it hard. Takes it for
a sin. For a long time he won't have nothin' to say to nobody. Just walks aroun' like he
don't see nothin', and he prays some. Took 'im two years to come out of it, an' then he
ain't the same.

His guilt
changed him and directed the rest of his life.