In Native Son, fear motivates Bigger Thomas to murder Mary Dalton. Even while Bigger is driving Mary, he feels uneasy in her presence. This uneasiness stems from Bigger's knowledge of racial stereotypes that suggest white women are the sexual targets of black men. Further, Mary's political talk makes Bigger feel afraid--he does not want to be falsely accused of sharing her beliefs and he does not want to have any knowledge of her beliefs. When Bigger takes Mary up to her bedroom, these fears come with him. He feels that he will be accused of trying to rape Mary if someone finds him in her room, and he knows that no one will believe him if he tells them that Mary was drunk.
The "real" fear that Bigger experiences stems from institutionalized racism--Bigger knows that to others in society he is immediately guilty because of who he represents, not because of who he actually is. These deep-seated fears cause him to act irrationally and murder Mary.