From what I read many Russians were hoping Germany would save them from Stalin and communism. But then realized that it was better to fight for the tyrant who would persecute them than the tyrant who would simply kill them.
Joseph Scholmer was a German Communist and anti-Nazi activist who was eventually imprisoned by the Nazis. After the war, while living in East Germany, he was arrested on a trumped-up charge and wound up in Vorkuta, a Gulag camp in the arctic and later recounted his experience in the book Vorkuta (New York: Holt, 1955). He wrote that had the Nazis announced that they were coming to liberate the Soviet Union, four million Soviet citizens would have joined them and Hitler would have won the war.
The Ukrainians eagerly joined the Germans. The Ukrainian Nazi police were sometimes called “Nightingales” because of their spectacular singing skills.