This is a great question. There are many examples of betrayal. In light of this, let me name a few of them.
First, the most obvious betrayal was when Amir allowed his faithful friend Hassan be harassed and raped by the town bullies. Amir knew that this action was wrong, but he could not muster up the courage to help Hassan. In many ways, this guilt lingered and festered, until his redemption at the end of the book.
Second, Amir also betrayed Hassan by accussing him of theft, which led to his dismissal from the house.
Third, we see a betrayal of the culture of Afghanistan in the take over of Russia and the Taliban. What was once loved and cherished is no longer, especially to people like Baba.
These examples should get you started.