Post traumatic stress seems to exist when someone is trying to incorporate, assimilate or digest the memory of a horrible incident into their system.
When we live our daily lives, we process what happens to us with knowledge that we have gained from our past up to the present point. We are able to assess, understand and respond to a given situation based on the wisdom gained from our past experiences. When something noteworthy happens to us, we will probably incorporate some interpretation of this into our system for future use. People seem to receive lasting psychological damage when something occurs that is horrible and that we can't understand. One question that can come up is, "What does this mean?" Another is, "Why did this happen to me?" And we may ask, "Will this happen again? ...Will I be ready for it?"
PTSD can happen when a good person has an outrageously bad experience. It is probably an experience that the individual was not prepared for in his or her upbringing.
In my past I have experienced a few incidents that left me traumatized, but that were less difficult than serving a tour of duty in which there is combat. I was locked in a supermarket overnight with armed robbers. I had been employed as a janitor, cleaning and polishing floors at the Flair supermarkets that once existed. The robbers had hidden in the back of the store on the same night that I was working, and came out of hiding too late; the management had already left. Had I been able to open the safe for the robbers, I might very well not be alive today. Instead, the store was robbed the next morning upon arrival of the morning crew, with me assisting at gunpoint.
Whenever my wife and I go grocery shopping at night, the memory comes back, and I become frightened that the store will be robbed.
Post traumatic stress is an emotional and spiritual scar that doesn't go away. I am skeptical of the claims of military officials who say PTSD can be treated to the extent that a soldier who suffers from it can be returned to active duty. If one night of terror left me permanently scarred on an emotional level, imagine exchanging gunfire over a period of months or years, being treated with medication and therapy for the trauma, and then being forcibly returned to the same horrible situation. That sounds like the epitome of abuse.
I haven't studied the recent shooting in Afghanistan. However, keep in mind that the military teaches people to fight using firearms. This fact along with PTSD is enough to create a shooting. If we were dealing with a nonviolent individual, you could claim that PTSD alone is not enough to make someone shoot innocent people. However, in order to make a soldier, this person has been taught to kill in the name of protecting his country. Since he has been given training to kill by the military, his self protection reflexes will probably not be nonviolent. This is different than someone who has never touched a gun and who never had to shoot to defend their life. Someone like that would likely not shoot people from PTSD, because it is not taught to that person as a reflex.
If someone has gone on multiple tours of duty and has been made to kill people day in and day out, then an "improper" shooting is not that much of a stretch. The government is saying to that soldier that killing some people is ok and is required, while killing against instructions is murder. Do you see the hypocrisy in our system?
I believe the system we have created is the true perpetrator, not the soldier who pulled the trigger. That soldier is an additional victim whose life has been ruined by his decision to join the military and serve his country.