Richard Wright’s Native Son is about a young black man named Bigger who accidentally kills a white woman. Most of the story takes place as Bigger tries to evade capture.

The first part of the novel presents a symbolic situation that foreshadows much of the rest of the novel. Bigger’s family lives in a one-room apartment. The story’s first scene presents a rat that terrifies Bigger’s mother and sister. Bigger and his brother then pursue the rat, which Bigger finally kills with a skillet.

Bigger’s own fate is similar to that of the rat. As the rat repulses the occupants of the tenement, so too does Bigger repulse white society after they learn of his part in the death of the young white woman. Bigger, like the rat, struggles to escape from those who seek to do him harm. The rat finds himself trapped with no avenue of escape. Bigger encounters the same situation when he tries to escape the police.

The turning point for Bigger Thomas in Richard Wright's "Native Son" occurs when the family's apartment is invaded by a rat. It is a disgusting spectacle as Bigger attacks the pest with a frying pan: "...it reared once more and bared long yellow fangs, piping shrilly, belly quivering."

Bigger finishes the rat off by pounding in its head with a shoe. Bigger, in typical big brother fashion, scares his sister Vera with the disgusting remains. She faints, and his mother scolds him, "Bigger, sometimes I wonder why I birthed you." Bigger responds, "Maybe you oughtn't've."

The rat and his mother's response to him is the proverbial last straw for Bigger, whose poverty and reason for being alive are equally unknowable.

What causes Bigger to view his family's tenement with disgust in Native Son?

When Bigger sees the opulence of the Dalton mansion, he realises just how "down and out" his family is. In his attempt to better himself (Here, note the connotation of his name....), Bigger takes some distance between himself and his family.  He is deluded into believing that he can somehow penetrate and become a part of the white man's world:

It is the image of Mary in a newsreel that inspired Bigger to take the job so that he might be closer to whites. He decides that by proximity he might learn how they make all their money. The film encourages him to pursue the American dream even though he is already excluded from it.

-from eNotes.com/native-son/themes

In a predestined way, Bigger's false aspirations lead to his downfall.  By venturing into the white man's world, he becomes increasingly vulnerable to it and finally panics when the pressure is on. He does not intentionally smother Mary to death but only covers her face to keep her quiet.  This act is provoked by the "natural" assumption her mother would have made that her daughter was being sexually aggressed. Note that this is Bigger's presumption of a presumption, but in this respect he was probably right.

In a way, Bigger sets his own trap, but he has been conditioned to react defensively, even in the absence of a real threat.  The mere dread of facing the supposition of guilt is enough to set off the domino effect of violence, and Bigger's downward spiral thereafter is inevitable.