Baba is a strong character in Khaled Hosseini's The Kite Runner, a man who is not likely to admit that he has any faults or frailties. In fact, however, to some extent he is to blame for many of the awful things that happen in this story. Though Baba does have a moral code, of sorts, his "sins" are significant.
Baba is known as "Mr. Hurricane," and that is an apt description of him. While he is able to move people and get things done, he is also an unstoppable force which does not lend itself to personal relationships and leaves a disaster in its wake. He claims to be a devout man and has a clear moral code; however, in many instances that code applies only to others and not himself.
The primary sin Baba commits is being a detached parent to his only son, Amir. A boy's relationship with his father is important in any case, but since Amir does not have a mother it is even more important for the boy to have a connection to his father. Perhaps even worse than his detachment are Baba's unreasonably high standards and expectations for Amir. The boy is always trying to measure up, to live up to his father's expectations, and he constantly fails. There are only certain ways Baba is impressed with Amir, and Amir spends most of his time and energy trying to accomplish them.
It is this incessant drive to win his father's acceptance, at least in part, which causes Amir to walk away from Hassan when he is being assaulted by Assef. In the end, that choice is as much about needing the blue kite to please Baba as it is about Amir's mixed feelings about Hassan--which were also caused by Baba. It is true that Amir makes the choice, but his conflict is created primarily by his father.
Baba is also a hypocrite. While he preaches to Amir that the worst sin of all is lying, Baba lives his entire life with the huge undisclosed lie of omission that Hassan is also his son.
“When you kill a man, you steal a life. You steal his wife's right to a husband, rob his children of a father. When you tell a lie, you steal someone's right to the truth. When you cheat, you steal the right to fairness.”
To add to the hypocrisy and insult, Baba has a relationship with Hassan that Amir covets, stirring up all the jealousy that eventually causes Amir to allow Hassan to be abused and to mistreat Hassan himself. Again, Amir makes his own choices based on what has happened to him, but it is certainly Baba who creates so much conflict for Amir. Amir even says their family friend Rahim Khan "rescued" him, and the one from whom he needed rescuing was, of course, Baba.
In Afghanistan, Baba attempts to atone for his sins by the things he most values: money. He builds orphanages and hires a surgeon to repair Hassan's hare lip, for example.
In America, Baba does seem to soften his moral position, and his rather pious proclamations are more like sage advice from someone who has made his share of mistakes. When Amir graduates, Baba tells Amir that he is proud of him, and that is clearly something Baba would not have done in Afghanistan. He also seems to be trying to atone for his past by arranging Amir's marriage in a way that suits Amir's wishes more than his own expectations.
Things do seem to get better once Baba and Hassan get to America, and Amir is able to eventually forgive his father for these things.