Why could Bigger never tell why he had killed in Native Son?

In Richard Wright’s novel Native Son, the protagonist, Bigger Thomas, cannot admit to killing his employer’s daughter Mary, nor to killing Bessie, because of fear of racist retaliation.
Throughout the novel, Bigger describes having desires to rebel against powerful sensations he feels. These sensations can perhaps be understood as reactions to a racially oppressive society operating on fear.
After he kills Mary, the narrator reflects on Bigger’s position in this society:

Though he had killed by accident, not once did he feel the need to tell himself, that it had been an accident. He was black and he had been alone in the room where a white girl had been killed: therefore he had killed her. That was what everyone would say, anyhow, no matter what he said.

Bigger relays that, primarily, he cannot confess to killing Mary because it will be seen as a crime, but in an underlying and secondary way, he cannot confess to the crime because it could confirm the fears of an oppressive white supremacist society.
The novel further illustrates this fear as the narrator continues to reflect on the killing:

To Bigger and his kind, white people were not really people; they were a sort of great natural force, like a stormy sky looming overhead or like a deep swirling river stretching suddenly at one's feet in the dark.

In this passage, the novel seems to be getting at an underlying sense of predetermined destiny when it comes to race, fear, and Bigger’s individual narrative. The “natural force” of the killing, the accident of it, is also the feeling that Bigger is trying to run from and that eventually leads him to kill Bessie.
Bigger cannot confess to killing Mary because the natural force will catch up with him, and he knows he will be convicted and killed himself.
http://www.academypublication.com/issues/past/tpls/vol01/12/28.pdf