Assef, Wali, and Kamal run into Amir and Hassan at the beginning of the novel, and Assef threatens to harm both boys. However, Hassan is carrying his sling-shot with him and defends himself and Amir by threatening to shoot Assef's eye out. Amir and Hassan leave the scene unharmed, but Assef promises to get revenge. Following the kite-fighting tournament, Hassan is cornered by the three boys after he retrieves the blue kite. Amir searches for Hassan, and witnesses Assef raping his friend as Wali and Kamal hold him down. Even though Amir has an opportunity to intervene and stop Assef, he does not come to Hassan's aid and chooses to hide behind a brick wall. Unfortunately, this moment is the turning point in Amir's life. His inaction and unwillingness to help Hassan haunt him for the rest of his life until he is able to redeem himself by saving Sohrab.
In the Book "The Kite Runner" Assef is a socio path who torments the children of the neighborhood. He is from a wealthy family and lives in Kabulwith Hassan and Amir. One day Assef and his friends confront Amir and are going to beat him up. However, Hassan steps in and pulls out his sling shot.
The second incident is far more serious. Hassan had run aftr the kite for Amir when he was stopped by Assef, Wali, Kamal, and Assef. Assef is hateful and has a grudge against Hassan. Assef had removed Hassan's pats and Wali tells him that what he is about to do is a sin. Kamal looks away from the scene.
Assef tries to convince the boys that raping Hassan would not matter because he is just a Hazzara. Assef then rapes Hassan. Amir is hiding but can see what is happening. He is too afraid to intervene and he bites down on his arm. It is the moment that changes everything in his and Hassan's life.