When Amir initially comes face-to-face with Assef in Kabul while he is attempting to save Sohrab from a terrible life of abuse and poverty, he does not immediately recognize his nemesis. However, Amir does notice a splotch of dried blood on Assef's white shirt and track marks on his forearm. Amir then mentions that he has seen those same track marks on the forearms of homeless people living in San Francisco. The track marks indicate that Assef has been using drugs intravenously. Afghanistan is renowned throughout the world for its opium production and heroin trade, which is more than likely Assef's drug of choice. The fact that he does not change his blood-spattered clothes reveals that he is proud of his violent work. Assef is a maniacal, psychopath, who proudly wears the blood of others and is probably trying to shock and intimidate Amir.
Assef may come from a wealthy family and, in the past, may have been a meticulous dresser, but his decision to join the Taliban meant that a few of his habits had to change. He is proud of the Russian jailer that he killed, and brags about the many Hazaras he murdered in Mazar-i-Sharif. He is probably equally proud of the stoning death in which he earlier participated, and he wears the blood as a badge. He may also have deliberately failed to change his clothes in order to further shock Amir. As for the track marks Amir saw on his arms, they were probably from injecting heroin or another drug.