I'd say anyone who chooses to kill a civilian, whether based on an order or not, is a war criminal. I really don't care if they thought they were there for a noble cause or not.
What if the civilian they kill has a substantial effect on the enemy's military capabilities, for example by being someone famous who loudly endorses the honour of signing up to fight for one's country, thereby motivating thousands of young men to do just that?
Once you open the door, where do you draw the line?
It is precisely the concept of such a "hard line" -- which by design enables such inhumanity to be passed off as inconsequential -- that must be brought out into the light and examined.