I have a friendship with a Vietnam vet. He was a chauffer for some American Officers during the Vietnam conflict. He was a heavy drinker and he still is to this day. He was attacked outside of a bar by a much smaller Vietnamese man who stabbed him three times in his chest. He was able to wrest the knife away from his attacker and he killed him. The doctors said of one of his wounds if it had been a quarter inch deeper the small man would have killed him.
He can't get over it. This person was the one and only person he ever fought much less killed. He's a drunk. He can't get the incident out of his mind.
It wasn't his fault so why does it bother him so much after so many years?
Is this common?
Both someone trying to kill you and killing someone are very traumatic experiences, and paradoxically almost the same thing due to these pesky mirror neurons. We're fundamentally social animals and no amount of indoctrination or drilling will make it otherwise. But, of course, different people have different breaking points.
At any rate, it's been known for a long while. In WW1 it was called "shell-shock", for example, and it was recognized as being pretty widespread.
We don't really know much more from before that, as basically WW1 was the turning point where people started figuring out soldiers as they really are, rather than as some idealized perfect warriors. Even in WW1, long after the soldiers themselves had started figuring out that some of them are cracking down under stress, it took a while for the command to really accept it. At first it was thought it was some physical effect of something new, e.g., the artillery shelling which was being used to unprecedented extremes. (Hence, "shell shock".) E.g., maybe carbon monoxide poisoning from the explosions or something.
Even then, anyone fleeing the battlefield because of it, was invariably court-martialed for desertion. And generally having it on the battlefield was most of the time classified as cowardice, so we may not even know how many had it but preferred to wake up screaming another night than be labelled cowards.
We can speculate though about the high numbers of desertions documented after major battles in middle ages and antiquity, even if a lot of them were forfeiting payment for their service or even forfeiting their privileged social class.
E.g., off the top of my head after the siege of Leontini in the second punic war, they found (and executed) about 2000 Roman deserters in the city. You'd think the Romans were all fearless and born with a gladius in hand, listening to their poets and orators, but reality seemed to have been very different. And bear in mind that those were at the time wealthy enough Romans to be eligible for the army, those who could afford their equipment, typically farmers owning a farm or the like. I'd guess it takes a lot of stress to forfeit it all and flee to a foreign city than fight another day.
It's easy to ascribe it to simple cowardice -- as it _was_ explained until WW1 -- but really one has to wonder. But, at any rate, it's hard to find an actual description of a syndrome or something. Just hundreds of thousands of "cowards" scattered across human history.
At any rate, sorry about the dry text above. I'm really sorry about your friend.