How has Assef changed since amir last encountered him? What is the significance of the story that he tells Amir?

Amir did not recognise Assef at first, between them the tall Talib in white, still wearing his dark John Lennon glasses, looking ike some broad shouldered,NewAge mystic guru....Assef talks about killing masses of people. The significance is that Amir was not there, he had escaped to America.