In "Ambush" by Tim O'Brien, a young Vietnamese soldier walks by as narrator keeps watch.

The moment is difficult for the narrator because he has to kill another human being. In essence, he doesn't respond to the difficulty, he doesn't have time to respond. There is no thinking, as he was trained to do exactly what he did..... throw the grenade and insure the safety of his brothers. He responds this way because there is no other way..... it's kill or be killed. Thinking will likely find you being sent home in a body bag.