In The Kite Runner, why did Assef have the courage to violate Hassan?

Assef's rape of Hassan was not an act of courage, not in
any way. It was an act of sadistic brutality and violence, an assertion of his power.
Assef is an arrogant bully, born of privilege, who preys upon those whom he deems
inferior. There is no courage in Assef. He has no conscience, and he has no humanity. He
does not hesitate to attack Hassan because Hassan is cornered and defenseless, and since
he is Hazara, Assef sees him as subhuman. He knows no one will hold him accountable for
his actions against any Hazara. Assef enjoys hurting and humiliating Hassan,
overpowering and dominating him, because Hassan had stood up to him earlier to defend
Amir. Assef is in fact a coward; he attacks Hassan when he has two friends with him
to gang up on Hassan.