Apparently the Germans and Russians were equally culpable of disposing of the Poles.
There is a section in “The Rise and Fall of the Third Reich” that tells a grisly tale.
Otto Ohlendorf, Einsatzgruppe chief, recounts the massacre of tens of thousands.
It is a tough read.
There are lots of complaints about the uncooperative actions of the Polish police or Polish civilians: knowing a ghetto was about to be deported, all the Polish police would call in sick. Some heart-rending stories about Poles married to Jews who chose death with their spouses rather than leave them or Poles who hid Jews or fed Jews hiding in the forest (an offense punishable by death).
I've never met a Pole who lived in Poland during the war who couldn't rattle off a catalog of horrors that would make your hair stand on end. My mother in law, for example, who was forced as a child to watch the Germans machine gun 300 civilians in a reprisal action-- she said she never would have imagined that human blood could run like water through the gutters.
There is a completely unknown episode from 1937 when Stalin order the elimination of Poles living in the Western Territories of the Soviet Union. 110,000 people were murdered (and possibly as many as 150,000). A book about this crime has just been published in Poland.