This is the best answer we'll probably get. After speaking with hundreds of WWII vets and discussing this topic, among many others, my impression was that in some units, there was a high moral ethic and it was highly punished if you took the discretion to shoot prisoners, while in other units, leaders had either an "I don't care" or encouraging policy towards the practice. Whatever the attitude of the commander, it, of course, filtered down to the troops, as many/all leadership qualities do.

And, of course, situations will dictate certain necessities as well as the attitude of the front-line troops.

My uncle served in the 90th Infantry Division in Europe, and most of his memories of the war are grim and painful to him. He told me that after he watched his best friends head disintegrate from suffering a direct hit from an anti-tank gun, while his friend was less than 20' away from him, one's emotions of anguish for friends skyrocket while one's humanity for the enemy disappears.