Had stories of the fates of captured German soldiers somehow made it back to Germany?
How did the common soldier know that the Soviets were going to be worse than the French, whom they also invaded, or the British, whom they had bombed?
France, Germany, and the United Kingdom had all signed the (1929) Geneva Convention, which guaranteed various protections for prisoners of war. For example, prisoners of war camps and food had to be a similar quality to the camps of the host's own soldiers, and seriously ill PoWs had to be repatriated home.
The USSR didn't sign the Convention at that time, so there were no protections for USSR soldiers captured by the Germans, nor protections for the Germans captured by the USSR. Similarly, there were no protections for anybody against the Japanese, because Japan wasn't a signatory.