O'Brien's daughter is only nine years old when she asks her father that painful question. I believe that the taking of someone else's life is such a life changing event that it should only be discussed by mature adults. If O'Brien chooses to tell his daughter the truth when she is older, that is okay; that is what I would do.
Later in his life O'Brien thinks back and tries to sort out the mixed up issues of the foggy dawn in Vietnam. Sometimes he believe that the young man would have just kept walking and never seen him, but sometimes he believes he did the right thing and forgives himself.
Although I don't know any war veterans, and I have never killed anyone, there was an event in my life similar to the one in this story. When I was in third grade my dad was electrocuted and actually was clinically dead. He was later revived and when my mom talked to him the hospital he told her some far out things. She has briefly mentioned some of them to me throughout my life, but I haven't and probably never will ask my dad what happened. It is just something that I don't think anyone wants to talk about.
Many people face many different tough situations throughout life. Few are as serious as killing another human being. The most difficult situations to understand are often involving death of some sort.