the process by which Ishmael undergoes changes and develops negative behaviors?

That could be a summary of much of the book. Here are a few suggestions:

Mattru Jong is attacked and Ishmael is forced to wander the countryside stealing food from people even more vulnerable than him.

Ishmael is recruited to join the army. He is given training and a gun to fight the rebels.

Ishmael is sent to a UNICEF camp but cannot reconcile his past with being in the camp.