Why does Jim Casy accept the blame for Tom when he attacks the police officer?

The sudden outbreak of violence is not an unexpected event, considering the previous accounts of the California deputies' cruelty and Tom's warning that he is still capable of committing violent acts. Yet the fight is somewhat softened: Tom does little more than trip the deputy, while Casy knocks the man unconscious. It is the deputy who causes the real havoc, inadvertently shooting an innocent woman. Still, the outcome of the event is significant for Jim Casy. He takes Tom's place as the scapegoat for the crime, sacrificing himself to save Tom. His role in the novel as a spiritual martyr is fulfilled.