For individuals killing is incredibly rare. Most people will never take a life, and even historically haven't. It also of course doesn't follow that merely because killing happens it doesn't cause moral injury, it's obviously possible for an event to be both common and traumatic.

It is generally accepted that most people will resist to kill and moral injury is very well documented for example in police who have to use deadly force so the result here is actually surprising. Afghanistan might also be an unusual case by historical standards, because close combat was very rare and the fighting much more impersonal.