Redemption is absolutely one of the central themes of The Kite Runner. The novel explores the subject in several ways, some less obvious than Amir's guilt towards Hassan—it runs through almost everything the characters do.
For example, with the exception of Soraya, you could say that most of the people the novel shows us start off fairly passive in regards to their own life. In addition to redemption, one of the main themes of the book could very well be cowardice, or peer pressure. It makes Amir and Baba and Rahim Khan, to note just a few, stand aside and continually make bad decisions even if they know in their hearts that they are making a mistake. Instead of acting according to their own will and conscience, they choose to hide, trying to fit into an unjust society.
This idea of trying to follow someone else's code of ethics and behavior goes quite deep. You could say it highlights the true essence of redemption: through the course of the novel, influenced by the events that shape his life, Amir comes closer to the realization that redemption is—and can only be—an internal freedom. It is not something that can be bestowed by others onto him. Forgiveness is a powerful tool, but I think that Rahim Khan is quite wise when he invites Amir back to Afghanistan. He does not offer Amir an easy answer, nor does he offer to pardon him of his sins against Hassan. What Rahim Khan says is that there's a way to "be good again," a way for Amir to redeem himself before the only judge that can truly liberate him—his own conscience.
With that, Hosseini shows that like loyalty, redemption has a worth of its own. It is nobody else's business and not theirs to bestow. Amir, finally gathering the courage to do the right thing, gets a chance to come full circle with his life. The novel sets up a very poetic salvation for him: saving Hassan's son from Hassan's fate, from the same bully who tormented Hassan.
It is significant that although The Kite Runner is full of powerful external events and misfortune, the one who hurts Amir most is Amir himself. The guilt he carries with him is not kept burning by anyone but him, and it takes him a long time to see that he needs to become proactive in order to achieve redemption. Only when he's done that, does he feel the weight lifted from his shoulders:
I ran. A grown man running with a swarm of screaming children. But I didn't care. I ran with the wind blowing in my face, and a smile as wide as the Valley of Panjshir on my lip. I ran.