As the previous post mentions, Kamal and Wali are Assef's friends. They seem to share some of his hatred for the Hazaras including Hassan. When Hassan chases down the blue kite that Amir cut out of the sky, Wali and Assef help to chase him down and are more than ready to beat him or try to get the kite from him.

They are less willing to go along with Assef when he decides to rape Hassan but they do so reluctantly. This terrifying scene is where Assef shows that in fact he is a sociopath but it also demonstrates the horrible willingness of people to go along with things they know are wrong when they are afraid or trying to fit in.

It is perhaps karma that later in the story the reader learns that Kamal was also beaten and raped when he and his father were attempting to escape to Pakistan.