Khaled Hosseini's novel about a boy who witnesses the rape of his closest friend and does nothing to intervene on his friend's behalf is indeed a morality tale. The Kite Runner is narrated by Amir, whose father is relatively prosperous and near whose home Hassan lives with his father while the latter two work as the family's servant. While Hassan is Amir's closest friend and his protector, Amir harbors deep-seated animosities regarding his father Baba's relationship with this poor servant boy. Hassan, it is revealed, is everything Amir is not: resourceful, brave, loyal. As Hosseini's story begins, a grown Amir is reflecting back on his childhood in Afghanistan with Baba and Hassan. These memories are triggered by a phone call from a friend in Pakistan. Amir instantly knows that this surprise call is no simple opportunity to reminisce. He instinctively understands that this phone call is about that event on that fateful day of the kite competition. As Amir notes regarding the meaning of the phone call, "It was my past of unatoned sins."
His failure to muster the type of courage more characteristic of Hassan to intervene while the latter was brutally sexually assaulted is the demon that eats away at Amir's soul every day of his life. He has kept buried deep within himself the secret that he witnessed that awful event and did nothing:
"I watched Hassan get raped," I said to no one... A part of me was hoping someone would wake up and hear, so I wouldn't have to live with this lie anymore, but no one woke up and in the silence that followed, I understood the nature of my new curse: I was going to get away with it... That was the night I became an insomniac."
Hosseini's novel is about the consequences of going through life with an act of betrayal as the defining characteristic of one's life. The Kite Runner most definitely allows for a thesis centered on morality. A thesis statement for such an essay, then, would essentially borrow from the above description of the novel's plot and from the quotes that illuminate the depths to which Amir's being has been affected by a childhood lapse in judgement. In that one moment of his and Hassan's lives, Amir made a decision that radically influenced the nature of his existence for the rest of his life. By choosing not to come to Hassan's assistance, Amir has not only betrayed his closest friend, but himself as well.
When writing a thesis statement for any paper it is important to remember to keep it clear and concise. Also, remember that the thesis statement serves as the "backbone" of your paper so it should address the writing prompt as well as serve as a guide for your reader to easily follow your argument. Lastly, it is also important to avoid using first person point of view in your thesis statement. Therefore, you should not use phrases such as "I believe", "I think", or "In my opinion".
The other part of your question states that you have the theme of morality. When writing about theme, it is important to remember that the concept of theme cannot be stated in one word. This is a topic or motif. Theme is the message an author is trying to convey to his or her audience about society or humanity in general. So, without knowing much more about the assignment, I would suggest thinking about what you think Hosseini is saying about the concept of morality through his novel, The Kite Runner.
So, an example of a thesis statement could be: Khaled Hosseini's novel, The Kite Runner, demonstrates how a person's morality develops over time based on his or her circumstances.