Hosseini does a very commendable job in bringing out how some of the worst political leaders started at a young age being some of the worst human beings. Assef fits this characterization. His attitudes towards the Hazara and Hassan when the three of them were young reflect this. His intimidation reaches an savage depth when he sodomizes Hassan in the alley. When he gives Amir a copy of a biography on Hitler for Amir's birthday, there is a clear insinuation that Assef will grow to "do something" with a great deal of anger and cruelty that is within him. The Taliban is a perfect match for this composite of character:
The events of the novel occur against the backdrop of political change, culminating in the rise of the tyrannical Taliban government in contemporary Afghanistan. Assef, Hassan's rapist and the bully who becomes a high-ranking Taliban official, embodies the consequences of the abuse of power for power's sake and the violence and repression of the Taliban regime. Assef is a sociopath who thrives in an atmosphere of chaos and subjugation.
In this light, Assef's work in the Taliban is a logical extension of his character as a young person.