Kamal dies badly, but there does not seem to be any suggestion that he has died in retribution for his participation in the rape of Hassan. He is not exactly an "innocent" victim, but still, more a victim of circumstances.
The rape scene shows clearly that Kamal is a follower, not necessarily evil, but certainly having chosen to follow the evil Assef. However, Kamal, when asked to actually rape Hassan by Assef, does decline, limiting himself to helping hold Assef down. This is does not make him a wonderful person, but it does show that he has his limits.
Kamal's death occurs in Chapter 10 of the novel, as Baba and Amir are fleeing Afghanistan for Pakistan. They, along with Kamal and his father, are being transported, hidden in a fuel tank. When Baba and Amir arrive in Pakistan and get out, they find Kamal's father, holding Kamal's dead body. He has died from the fumes in the tank. Kamal's father grabs the gun of Karim, who is the smuggler transporting all of these escapees. Karim believes Kamal's father is going to shoot him, but Kamal's father turns the gun upon himself, no longer wanting to live after the loss of his son.
This is not a central part of the novel, to be sure, but it does provide an additional element to the theme of fathers and sons and also tells us that sometimes it is too late to have "a way to be good again" (2), a chance that Amir is afforded.
kamal will emerge more visibly as the feeble, pathetic person he is, ultimately dying of asphyxiation, which so saddens his father that he commits suicide. Kamal, in short, is a minor character, but one whose decline and eventual death lends this story an added poignancy.