How do kite flying and kite fighting develop the themes in the novel?

In The Kite Runner, the kite is both a positive and negative symbol. As such, kite flying and kite fighting simultaneously trace the themes of betrayal, generational conflict, and redemption as the story progresses.
Amir, ever cognizant of his fragile relationship with Baba, takes up kite-flying in his childhood years; it is the one activity that allows him to relate to Baba on a personal level. As a former champion kite fighter, Baba is partial to the sport; Amir sees it as the only means by which he can redeem himself in his father's eyes for his lack of prowess in other, more tactile sports.

Baba and I lived in the same house, but in different spheres of existence. Kites were the one paper thin slice of intersection between those spheres.

Preparation for each year's kite-fighting competition can be surprisingly brutal on young hands. Amir relates how "by the time the snow melted and the rains of spring swept in, every boy in Kabul bore telltale horizontal gashes on his fingers from a whole winter of fighting kites." As a warring sport, kite-fighting also symbolizes the inner conflicts in Amir's life. For example, the blue kite represents Amir's abandonment and subsequent betrayal of Hassan (it is while acting as Amir's kite runner that Hassan falls into Assef's hands and is brutally raped). Later, Amir basks in Baba's applause and admiration rather than admit his winning kite is tainted by Hassan's blood and pain.
The kite also develops the theme of Amir's redemption. By the time he flies the kite with Sohrab at the end of the book, Amir has redeemed himself. He has retrieved Sohrab (Hassan's son) from the orphanage, and, however clumsily, done his part in freeing Sohrab from Assef's cruel custody. In teaching Sohrab the principles of kite-fighting, Amir is gifting Sohrab with Hassan's legacy of selflessness and courage and redeeming himself of his past sins.

I did it perfectly. After all these years. The old lift and dive trap. I loosened my grip and tugged on the string, dipping and dodging the green kite. A series of quick sidearm jerks and our kite shot up counterclockwise, in a half-circle. . . "Do you want me to run that kite for you?" His Adam's apple rose and fell as he swallowed. The wind lifted his hair. I thought I saw him nod. "For you, a thousand times over," I heard myself say.