I really suggest reading On Killing by Lt. Col. Dave Grossman, US Army (Ret.).
Humans do not want to kill each other and doing so fucks them up more than anything you can imagine. Its not the fear of death, or the fear of injury that scars people in war. Its the fear that they have done something wrong, or will have to do something innately wrong, or that they will have let someone else down because they can't do what they feel is wrong (survivors guilt, etc).
It is even worse today because they are so conditioned to kill that it becomes a mechanical process, the scaring effects of battle not appearing till later in a way that makes them second guess themselves.
Also, especially in Vietnam, we do not take the proper measures to "deprogram" and re-assimilate and provide care for their mental wounds when they come back, we being not only the military but our society. It has gotten quite a lot better, but its still a very hard thing for these men and women to come back and not feel like they are out of place somehow, or guilty.