There were many bodies, real bodies with real faces, but I was young then and I was afraid to look. And now, twenty years later, I'm left with faceless responsibility and faceless grief. (Good Form.7)
Civilians might assume that ignoring the face and body of a man you've killed makes the guilt easier to bear, but O'Brien is saying the opposite. He has to give the dead a face and a past in order to cope with his guilt. It's why he created the young man on the trail to My Khe.