Germans were trying to exterminate poles/jews. If i remember correctly, poles were to be worked to death by 1975. Russians were just suppressing poles/jews, they did not have an extermination policy. Therefore they were "less evil."
No one knew at the time what Russians were doing to the Polish people, soldiers and the Polish Intelligence.
They've tortured and murdered Poles not only during the war but for many years afterwards.
Here are some of the facts:
"In 1939, during the Russian invasion of Poland, some 14,500 Polish officers were captured and interned in three P.O.W. camps in the Soviet Union. The next time the world heard of these prisoners was a news broadcast on April 13, 1943, from Radio Berlin. It stated that the German Army had discovered mass graves at Katyn, 18 kilometres north-west of Smolensk, near the village of Gneizdovo and containing the bodies of Polish officers. Eight graves were opened and 4,253 bodies exhumed. All were dressed in Polish uniforms, with badges of rank and medals intact. No watches or rings were found on the corpses. It was established that the bodies were of Polish officers from the camp at Kozielsk, situated in the grounds of a former Monastery, near Orel. Two other camps, at Starobielsk (3,910 men) and at Ostashkov (6,500 men) were wound up and closed in the first days of April, 1940. Whatever happened to these 10,000 odd officers has never been established. They were never seen alive again. From evidence obtained after the war, all prisoners of Kozielsk camp were shot by Stalin's NKVD.
On April 13, 1990, fifty years after the massacre, the USSR for the first time admitted its responsibility for the murders."
Link to more info :