suddenly, a rock struck Hassan in the back. We whirled around and my heart dropped. Assef and two of his friends ... were approaching us.
Assef's characterization is one of the strongest examples as to how childhood determines a future. As a child, Assef was a bully. He targeted Hassan for being different. Armed with his racial purity arguments and a misplaced love and reverence for Hitler, Assef as a child who loved power. He enjoyed being able to exert this power. The more he could victimize another person, the greater the display of power. When he sodomizes Hassan, it is a reflection of how much he enjoys power and the lengths he will go in demonstrating it.
This becomes part of Assef's motivation in joining to Taliban as an adult. The bully has become an integral part of an organization that lives off of intimidation and enforcing its will upon others. For Assef, the same traits he showed as a child are ones that he displayed as an adult in his participation as a member of the Taliban. He continues to enjoy victimizing others. In leading the public stoning of those who commit "sins' to Sohrab, who is the embodiment of human objectification, the child bully that Assef was has grown into an adult with unlimited power who thrives off of the suffering of others.