...in some cases I have heard of each executioner was so sure that another would fire the lethal round that he deliberately avoided doing so to not have it on his conscience
I've heard several scenarios on the firing squad process. One is that only one person has a loaded gun, but no one knows which one. That way most of them feel fairly secure his shot wasn't the fatal one.
The other version has all the guns loaded except one. That way they can all harbor thoughts that their gun was the empty one.
I have a tendency to believe the second one since in the first scenario, if the guy fires an errant shot.... then you'd have a real mess on your hands.