This is when Tim O'brien was talking to his 9 yr. old daughter Kathleen about the war. She asked him if he killed anybody because he is writing all these war stories and his response was no, because he didn't want her to know that, he wanted to so bad to tell her he did, that's why he keeps writing these stories.
After having that conversation with his daughter, Tim started to remember what happened the day when he killed that young boy. He was taking his turn guarding while the others were sleep. He saw a young boy and with all black on and sandals. He got scared and threw the grenade at the young boy, not knowing he wasn't in danger.
After throwing the grenade, he examined the boy and he realized he really messed up. His chin was in his throat, his eye had a star shape, and his legs were beneath him. He went over to the dead body and started thinking about what the boy probably would have become but he couldn't because he killed him out of fear.