I read few years ago in a New York Times story, that according to German Red Cross, 1.3 millions german soldiers, taken alive as Prisoners of War by the Red Army, are still unaccounted for, and the russians claims to have no records of whereabouts/fate of these german prisoners of war. This story in New Times was written, because a planted forest in one of the former Soviet states, was resently discovered to have the shape of an inverted swastika, as well as the age of this planted forrest was around 60 years old, and it was therefore speculated that this forrest was planted by german prisoners of war.
On the other hand, the germans captured about 4 million russian soldiers, and about 3 million of them died in german captivity.
The understanding I get from watching various Youtube film clips from war time Russia, is that there was a genuine Russian rage over the german invasion of their homeland, and they were well aware of the german attrocities against their countrymen, and the russians took revence when ever they could.