I don’t really blame any troops for executing partisans or civilians who take potshots at them. Can’t hide behind non-combatant status when you’re trying to shoot someone in the back. I’m a bit surprised that soldiers caught with shotguns were executed (like somebody said, in a world of mustard gas and flamethrowers...), but soldiers in every war have taken a particular dislike to enemies using certain weapons. All sides in WW1 executed soldiers with sawback bayonets, and snipers usually didn’t fare well either when captured.
I don’t think that they actually did execute soldiers captured with shotguns, but they did threaten to. I believe we let them know that we’d retaliate if they tried such a thing. WWI Germans were a bit tone deaf when it came to wartime PR. Their execution of British nurse Edith Cavel for helping British POWs escape and their execution of Captain Charles Fryatt who tried to ram a U-Boat as an “illegal combatant” didn’t win them many friends.