Why is the scene with the blue kite and Hassan's rape significant to the story.

Amir wins the kite competition with the blue kite: Hassan is his runner. The kite becomes a symbol of triumph. Later, Amir sees Hassan in an alley being confronted by the sociopathic Assef. Hassan is raped and Amir witnesses the blue kite falling to the ground. Amir does nothing to help his friend: the blue kite becomes Amir's greatest failure.