Epigraphs are like little appetizers to the great entrée of a story. They illuminate important aspects of the story, and they get us headed in the right direction.
Even today is my complaint rebellious, My stroke is heavier than my groaning —Job
The epigraph is a quotation from the Book of Job. Job was a good guy—faithful to God and wildly successful. He had a wife, wonderful children, massive wealth, and great health. However, Satan dares God to take all of Job’s blessings away and see if Job is still faithful after all the suffering. God agrees, and gives Satan freedom to do what he wants with Job... as long as he doesn’t kill him.
After Satan epically messes with Job’s life, Job sits there with boils on his face, stricken with poverty and hunger and pestilence and plague, his kids and servants dead. He questions why God allowed him to be treated like this. The only thing left to him is his wife, who grows bitter and encourages Job to rail against God. Yet even as he questions God, Job refuses to curse him. So in this particular game, God wins. Job struggles to maintain his spiritual faith in the face of tragedy and finally does it. He’s thus seen as one of the "righteous" men.
The quotation with which Wright chose to open Native Son comes from the period when Job is still frustrated about his sufferings. He’s still trying to figure out what’s happening to him. As Job begins to realize that his sufferings are due to forces he can’t control, he complains bitterly that he feels isolated, alone, and abandoned. Though Bigger isn’t "righteous" like Job, he is similarly a victim of larger forces that batter him about mercilessly—forces of racism, injustice, and poverty.
His bitterness and rebellion are natural responses to an unjust system. In the quote Wright used, Job hasn’t yet submitted to God; he's still questioning the justice of his situation and the goodness of a God who would use him to wager a bet. Bigger likewise questions the justice of his situation. Instead of submitting to a social system of racism, he rails against it in the only way he knows how. His murder, ironically, awakens him to the forces of racism that caused him to murder in the first place—and waking up to the truth makes him want to live.