This is a good question. Evil is manifested in several areas. Let me give you a few examples.
First, the most obvious place where evil is found is in the Taliban. They are portrayed as heartless, brutal, and unprincipled. They basically run the nation. The Solviet Union is also portrayed as evil as well.
Second, we also see evil in more subtle ways. When Amir is not able to match up to the courage and loyalty of Hassan, he treats him poorly, especially after this rape. This, too, is an manifestation of evil as well.
Third, we are also able to see evil when Baba moves to America. He is not treated with the dignity of his position in his home country, and he has a very difficult time adjusting to American life.
In the end, of course, there is redemption for Amir, which makes the novel end on a note of hope.