Tim O'Brien is a young man fighting in the Vietnam war. When he first got his draft papers, he went to the border of Canada and thought about not fighting. He didn't believe in the war, but knew that his guilt would of not fighting, would weigh on him for the rest of his life.
In the chapter, The Man I Killed, O'Brien talks about his guilt over killing the young man. He tries to humanize the man, by imaging what his life is like. He sees beauty in the surrounding area where the young boy lies dead.
"Frail-looking, delicately boned, the young man would not have wanted to be a soldier and in his heart would have feared performing badly in battle... He loved mathematics... and at school the boys sometimes teased him about how pretty he was, the arched eyebrows and long shapely fingers... Beyond anything else, he was afraid of disgracing himself and therefore his family and village."
In the chapter, Ambush, we get to see why O'Brien kills the young man. He and his platoon were sent to the village of My Khe, and in the early morning fog he sees the young soldier coming out. Without thinking O'Brien throws the grenade at the boy and knows instantly that he is going to die. O'Brien lives with this guilt all of his life. He killed the young soldier because they were at war. He was trying to protect himself and his fellow soldiers. Although O'Brien was trying to protect himself and others, he still lives with the harsh reality of what war made him do.