When O'Brien's daughter, Kathleen, was nine she asked him if he had ever killed anyone. She knew he had been a soldier because he kept writing so many war stories, which she thought proved he had to have killed someone in battle. He says it was a hard moment, but he told her that he hadn't. O'Brien notes that he hopes Kathleen will ask again when she's older, but he's using this story to pretend that she's an adult so he can tell her the truth, or what he remembers. Then he wants to tell her that as a girl she was right, and it's why he's continued to write war stories.
O'Brien is using this story as a vehicle to assuage his own guilt for not only lying to his daughter, but to try to put words to an event that continues to haunt him into the present. He wants to address this to his daughter as an adult so that she can know she was right in her youth, and even though she pokes fun at him for writing war stories all the time, he hopes this story will explain why it's so necessary for him to do it.
The young man was short, thin, and frail—about twenty years old. O'Brien was afraid of the man, and when the man passed him on the trail O'Brien threw a grenade that, when it exploded at the man's feet, killed him.
This is the recurring, nearly identical, description of the man who O'Brien killed. Its repetition throughout stories shows the way not just the fear of death but the aftermath of killing someone can haunt you.
O'Brien insists on going back further. After midnight, before he threw the grenade, the platoon moved to an ambush site outside My Khe. Everyone was there and spread out along the trail, hiding in the brush, taking turns sleeping and keeping watch. Kiowa was O'Brien's partner, and woke O'Brien up while it was still dark for the last watch. O'Brien, groggy and disoriented, lined up three grenades in front of him. The sun began to rise and the trail became more visible. He saw the young man emerge out of the morning fog, wearing all black and sandals, and carrying a weapon. O'Brien already pulled the pin on a grenade and was in a crouch. He did it automatically. He didn't hate the man, see him as an enemy, think of morality, politics, or duty. He tried not to throw up. O'Brien was scared. He wasn't thinking about killing; he was using the grenade to make the man leave, "evaporate." He remembers he had thrown the grenade before telling himself to.
O'Brien wants to set up a backstory, though, to give the reader and his daughter some perspective so they don't think he acted without cause. The way O'Brien describes throwing the grenade, and the decision (or lack thereof) highlights how thoughtless and automatic a decision in war can be—particularly for a terrified young man like O'Brien. O'Brien makes clear that he wasn't acting to kill the man, he just wanted him to disappear. There were no moral questions that made him pause before he threw the grenade, he was in a place of fear. Morality did not come into it. The instinct to survival overshadowed any moral compass.
O'Brien lobbed the grenade, and it seemed to freeze in mid-air. He ducked down and held his breath. He claims he didn't hear it land, but the young man must have because he tried to make a run for it. The sound of the grenade was like a pop, neither soft nor loud. The young man jerked up and then fell to the ground. His sandals were blown off his feet, his right leg was bent beneath his body, one eye was shut, the other was a star shaped hole.
Again, O'Brien repeats his description of the dead young man that he killed. The star shaped hole is used in multiple stories in the collection. It is these repetitive descriptions that make the man he killed haunt the collection as well as the reader, just as they haunt O'Brien.
O'Brien notes his life wasn't in immediate danger. It's likely the young man would have just kept walking. "And it will always be that way."
O'Brien acted from fear, and couldn't stop himself. Didn't even realize he was acting. In the aftermath he can understand the morality.
O'Brien remembers Kiowa trying to console him by saying that the young man would have died regardless, told O'Brien it was a "good kill." Kiowa said it was a war, O'Brien was a soldier, and that O'Brien needed to stop staring at the corpse and ask himself whether he would prefer to be the dead young man. Kiowa's words don't get through to O'Brien, though. He can only keep staring at the "fact of" the dead young man's body.
Kiowa's characterization of the kill as a "good" one shows how destroyed the lines between right and wrong are in war. Not only is killing acceptable, but there are kills that are deemed "good." For O'Brien, his guilt outweighs Kiowa's attempts at consolation. O'Brien is responsible for the fact of another man's death.
O'Brien says that he still struggles with the event all the time, going back and forth between forgiving himself and felling guilty. When things are ordinary, he tries not to think about it. Occasionally, though, when he's reading a newspaper or sitting alone he will see the young man again. He will step out from the fog in the morning and O'Brien will see him walk towards him. The young man will walk close to O'Brien and then smile to himself from a "secret thought." Then he will keep going down the trail and disappear into the fog.
Just as the war is contradictory, so are O'Brien's haunted feelings about the man he killed—sometimes he can forgive himself because he understands what forced him to act in the way he did, other times he knows that what he did was wrong. In his vision, O'Brien assuages his guilt not by trying to forgive himself but by making the young man live, as only a story can let someone live.