In the course of a fire-fight he killed one of the Americans who had attacked the village in which he was living and who had invaded the country in which he had grown up (insofar as a 15 year old boy can be said to have grown up). In the course of the fight, Omar was himself shot twice, and permanently blinded in one eye.
Sad, but in no way criminal, neither on his part nor on the part of the soldier who shot him. People get killed when they invade countries, and people get killed fighting against such invaders.
And all this has nothing to do with the rights and wrongs of the invasion.