Why did Amir feel it was either him or Hassan who had to go?

Immediately after Amir wins the kite-fighting tournament, he runs after Hassan and finds him surrounded by Assef and his two friends in a back alley. Instead of intervening and saving Hassan, Amir hides behind a cement wall and watches as Assef rapes Hassan. When Hassan returns home with the blue kite, Amir becomes overwhelmed with guilt and cannot look him in the eye or speak to him. Amir and Hassan's friendship is ruined after the traumatic event, and Amir feels like he can no longer be in the same home as Hassan. Amir goes out of his way to avoid Hassan and decides to set Hassan up by making it seem like he stole his birthday gifts. Amir then puts his gifts in Hassan's tiny shack, and Baba ends up finding them in Hassan's possession. Hassan accepts blame for "stealing" Amir's gifts, and Ali decides to leave Baba's estate even after Baba forgives Hassan and begs them to stay. Overall, Amir feels that either he or Hassan must go because he is overcome with guilt after refusing to help his friend in a time of need.