What is Amir’s reaction when Assef attacks Amir when Amir returns to Afghanistan?

Assef is a sociopath. He derives pleasure from hurting and humiliating other people. The day Assef rapes Hassan because of Amir's cowardliness is the day that will haunt Amir for the rest of his life. Amir finally has a chance at some redemption by reclaiming Hassan’s son Sohrab from Afghanistan. Here Sohrab becomes a substitute for his father. Sohrab is raped by the same man, Assef , that once raped his father. The conflict comes full circle. Hassan must face Assef again but must stand firm. Assef savagely beats Amir and Amir laughs during the beating; he is finally feeling a little redemption for his life-long guilt: he has waited for this beating for most of his life. The pain is overdue payment for his cowardice that fateful day.