Nothing about Assef is courageous. He is the worst individual antagonist in The Kite Runner by Khaled Hosseini, and he is a coward, a sociopath, and a bully.
Assef is a rich, imposing, blond-haired Pashtun boy whose father is an Afghan and whose mother is a German. He is raised in privilege and could have been and done anything good; instead he acts like the worst of privileged kid. Rather than counting himself blessed and making his world better, Assef abuses his position and connections to satisfy his depravities.
Nothing Assef does is courageous. He routinely taunts Hassan and, by association, Amir. He only rapes Hassan, a Hazara who cannot fight back, when Assef gets Hassan alone in an alley with several of his cohorts who, by the way, also think Assef is depraved. That is not a courageous act.
Later, when he comes to Amir's birthday party, he taunts Amir by giving him a biography of Hitler to make his point that the "impure" races (like the Hazaras) should be eradicated--and so should anyone who likes and protects them. Taunting is not courageous, either.
Much later, when Assef is a leader in the Taliban, he abuses--on a much larger scale--those who cannot defend themselves. He takes young boys from orphanages to be his sexual slaves, and he routinely kills Hazaras for fun. Amir has to listen to this gleeful, sick recounting by Assef before he can get Sohrab away from Assef:
He leaned toward me, like a man about to share a great secret. "You don't know the meaning of the word 'liberating' until you've done that, stood in a roomful of targets [the Hazaras in Mazar-i-Sharif], let the bullets fly, free of guilt and remorse, know you are virtuous, good, and decent. Knowing you're doing God's work. It's breathtaking." He kissed the prayer beads, tilted his head.
Assef is a sadistic sociopath who has no discernible redeeming qualities. Usually a villain has something to which we can relate or with which we can sympathize; however, Assef is a pure villainous character who had everything given to him to be a force for good but chose evil at every turn. Assef is not a courageous character.