Guys behind the lines to shoot anybody that Retreats from their position for any reason.

That's not what the purpose of blocking brigades was. Casualty rates were roughly similar between Germany and the USSR, if you don't include civilians.

There's a lot of fictions made up by nazis about the eastern front that western historians took seriously until the Soviet archives from the time were made public, that we still see in Hollywood and pop culture, like machine-gunning their own troops, sending soldiers without weapons, and human-wave "tactics".