I think as well we need to be aware of the role of Baba in the adult that Amir became. However noble a character, he struggled to accept his son for whom he was and for the gifts he had. As parents we constantly need to be aware of the danger of making our children in our own image, and I think Amir suffered because of Baba's expectations of him and the way that Baba couldn't accept him for who he was, even going as far as saying he doubted whether he was his son.
I do like bullgatortail's take on a theme for an essay. A few other ideas come to my mind, too.
I wonder to what degree the fact that Amir and Hassan shared a father shaped both of their lives. Hassan was a strong, loving, and contented child, who, in spite of his suffering in later years, appears to have been a well-adjusted and happy man. He had the love and approval, really, of two fathers. Amir, on the other hand, was constantly seeking love and approval in a "competition" he had no conscious knowledge of. This clearly affected him adversely, and it was not until he and his father emigrated to the United States that he was able to gain and feel some love and approval from his father.
Another theme you might explore is how early years in a war-torn and unstable country affect people as adults. I think that some of the insecurity that Amir held was because of this. This is an area of psychology that you could research because, sadly, we now have a greater understanding of what circumstances like these can do to people, damage that can last a person's entire life, with examples all over the world to choose from. Can we ever recover from such wretched early years?
I'm sure many people commit acts of which they are ashamed when they are younger, just as Amir had done to Hassan. And, just as Amir spends many years seeking redemption for his past mistakes, many adults also try to find a way to redeem themselves. There are many ways to try and "make things good again," from philanthropy (like Baba) to trying to lead a good and honest lifestyle. Few people, however, get the chance to make up for the past in the way Amir did.