What is the motivation behind Assef's abuse towards Hassan and why does Amir not help him???

Assef and the other boys are asserting dominace over the boy they consider nothing more than a worthless Hazara. Their attempt to steal the kite was child's play, the rape was an act of dominance.... a way to make Hassan subservient. Assef was the only boy to rape Hassan..... the other two boys held him down.