Stalingrad

I remember watching a movie in class about Stalingrad and the Red Army would actually kill their own men if they tried to retreat. Which I don't understand how they can kill the men that are running back to them, if they, themselves are just sitting there in the first place. Also why would you want to the killer of someone who is trying to help your side out...wouldn't the other guys killing them be enough.  Doesn't winning a war have a part where you actually need men...by the rate the Russian Army is going, that they are going to have any men left.  Another thing is, if you were to shoot someone that was on your side would you play it off, by saying that they were killing in battle? Like what if the same man that killed someone had to go to the family of the dead man and tell them that their father, husband, son,brother, uncle. or whatever he was to them was now dead.  Would that soldier have to confess that he in fact was the man that took his life.