I need more information with the conflict between Amir and Assef.

Assef is a sociopath. He derives pleasure from hurting and humiliating other people. The day Assef rapes Hassan because of Amir's cawardlyness is the day that will haunt Amir for the rest of his life. Amir finally has a chance at some redemption by reclaiming Hassans'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.