You have a great idea for your thesis, but your thesis statement needs to explain not only your thesis but also your supporting points. A thesis statement is a kind of "because" statement: This is true because of X, Y, and Z. While I do not know what supporting points you have thought about yet, I do have a few ideas on this, so let's look at those and then at an example of a thesis statement.
Your supporting points need to show what in the novel supports the idea that the pursuit of goodness is the key to personal freedom. There are some characters in the novel that support this idea, and you might choose three to make your point. The lives of Amir, Hassan, and Assef, in varying ways, show the truth of this statement. Amir cannot be truly free until he is able to rescue Sohrab, an important pursuit of goodness, to make up for all the times he did not pursue goodness in his past. Hassan, in spite of his tragic ending, does have personal freedom. He has a clear conscience because he has pursued goodness for his entire life. And we can see that Assef, who pursues only evil, is never truly free, entrapped by his evil ideology and quest for power, dying painfully as a consequence of his wickedness.
If I were writing my own essay on this wonderful book, I might want to write about how class and ethnicity play a powerful part in life, a thesis I could support like this:
In The Kite Runner, class and ethnicity play a powerful role in the lives of two families, those of Baba and Amir and Ali and Hassan.
I have stated my main idea, which is what a thesis is, and then I have explained how I will support that idea. This organizes my body paragraphs, too, one on Baba and Amir and the other on Ali and Hassan. Once I have written my body paragraphs, I can add a conclusion that reminds my reader what my main idea is and give the reader a little review of my supporting points.
You have a fine thesis idea, so what you need to do now is decide what points you will be making to support your idea. Then you can "list" those points in your thesis statement. Make your thesis statement the last sentence in your introduction, where it helps the reader to see what is coming, and where it makes a nice transition to your first body paragraph. Good luck to you with your great idea!