"The Germans who carried out this execution were most likely members of the Nazi Party but they were still Germans!"
Not necessarily - many security personnel seconded from the SS were foreign conscripts - usually, but not exclusively - from Eastern Europe and the Balkans who chose to fight for the Germans rather than suffer the horrors of the prison camps. These unfortunates were often used to carry out the dirty work during WW2, and as the murders were carried out in Eastern Europe then it is possible that they carried out the deed. Unlike the film, the victims were sent out in small groups of about 3-4 to be shot during a "rest period" in their journey. The bodies were burned and their ashes scattered to hide all evidence of the crime. The orders were undoubtedly of German origin however, and the greater responsibility lies with them.