He may not have known if he shot anyone or killed anyone. My work with Vietnam veterans with PTSD has made me realize that we imagine war violence as "I point gun at person, I shoot person, I see person die" but the Vietnam War - like many conflicts - was extraordinarily chaotic.

Combat often happened in the jungle. You might hear gunfire, so you return gunfire in the direction that you heard the gunfire. How do you know when the 'enemy' is dead? When there's no more returned gunfire - which is not always clear if there's a lot gunfire occurring from your side. Did you see people firing at you from somewhere the vast, dark jungle brush? Maybe. Did you clearly see specific people - like their faces, their bodies, their guns - shooting in your specific direction? Maybe, parts of them, sure, sometimes. Did you see a specific person through the jungle fall and die when you aimed and shot at them specifically? That's difficult to say even in the best of circumstances. If you can see him, then the guys in your unit likely can too, and they're going to be shooting at him, too. In all likelihood, in a significant number of fights in Vietnam, you're probably shooting in a general direction at people in the bush under high emotional duress, hoping to survive the next few minutes. Combat was often a unclear, chaotic mess. Think rapid flashes of pieces of visual information, not certain slow motion clarity.

Of course some veterans know that they killed people, because they shot them up close or saw them die in other ways. The standard "I point, I shoot, I see someone die" did happen in the Vietnam War, but many other situations happened too. All can create significant trauma for soldiers and veterans to develop PTSD.