Bigger Thomas's bitterness against the white man is the emotional foundation of Richard Wright's Native Son. It is not a personal grudge but the accumulated resentment of a young black man crushed by a racist society, and Wright gives it several clear sources.
Poverty and confinement. Bigger and his family live in a single rat-infested room in the Black Belt of Chicago, paying high rent to white landlords for slum housing. He sees that black people are deliberately kept poor and hemmed into overcrowded ghettos while whites live in comfort. This economic injustice breeds resentment.
Denied opportunity and blocked dreams. Bigger is intelligent and full of restless ambition, but every avenue is closed to him because he is black. He cannot become an aviator or pursue any of the futures he glimpses; the best he can hope for is menial work such as being a chauffeur. He is bitterly aware that the white world has stolen his possibilities and reduced him to a servant. As he watches a plane overhead he feels the ache of what he is forbidden to become.
The daily humiliation of segregation. Bigger lives with the constant reminders that he is regarded as inferior: the separate, restricted spaces, the deference he must show, the sense that whites do not see him as fully human. This ceaseless humiliation feeds a deep, smouldering anger.
Fear and a sense of being watched. Whiteness looms over Bigger as an all-controlling, judging force (symbolised by the blind, white-clad Mrs. Dalton and the pervasive whiteness of the novel). He feels perpetually observed, powerless and afraid, and this fear curdles into hatred of those who hold such power over him.
Even white "kindness" oppresses him. The liberal Daltons and the communist Jan and Mary mean to be friendly, but their attempts to treat him as an equal only make Bigger acutely uncomfortable, exposing how abnormal genuine equality feels in a racist world. Their patronising goodwill deepens rather than eases his alienation.
Conclusion. Bigger's bitterness springs from poverty, blocked opportunity, daily humiliation, fear and a suffocating sense of white control over his very being. Wright presents this bitterness not as an excuse for Bigger's crimes but as their social root, arguing that a society which cages and dehumanises its black citizens breeds the very rage it then condemns.