Can Killing be Justified?

For someone looking in from the outside, I am certain that they would find the recent New York Times article about the soldier who killed five of his comrades in a counseling center at an American military base. I believe, though, someone who is a soldier or has been a soldier in the war and fought has no trouble explaining how something like this could have happened. Even though I have never fought in a war, after our recent unit of War: What is it good for? where we watched some artifacts, I feel like I have more of a clue why something like this could have happened. In the movie Born on Fourth of July, Tom Cruise is fighting in the Vietnam War and while he is retreating from a village that is under attack he is shooting enemy soldiers. Over a ridge, outlined by the sun comes a soldiers, gun raised and shooting. With the camera angle shown (from the perspective of Tom Cruise), it is impossible to tell the soldier apart from the enemy. With only a split second to make a decision, Tom shoots the soldier, killing him. Later he finds out that it was one of his comrades that he killed, but there is no way Tom could have been blamed for it, because every other soldier in the same situation would have done the same thing. Although this case in the military base was more intentional, I think the same reasoning applies. The soldier was obviously affected by the war because he was in a counseling center, and knowing a bit about the subject because I wrote my junior theme on the topic, the patient probably suffered from PSD. Patients with PSD (post dramatic stress disorder) always seem on edge because they relate everything to war. For example, thunder could be mistaken for a mortar shell. This soldier in the counseling center could have associated something the five other soldiers had or wore as something the enemy would have had or worn, and thinking he only had a split second between life and death, acted quickly with his instincts-to kill. I do not think the soldier should be prosecuted or held accountable for his actions because he acted that way because what war has done to him, and I believe any other soldier would have acted the same way. I think the bigger issue/question is how was a mentally instable soldier in a counseling center have access to a gun? I think we need to be more aware of the possibility of something like this happening, and take actions to prevent it, instead of putting the blame solely on the soldier who made the misjudgment.