Stalin had been at war with the Ukraine for years and genocide was his way of dealing with them. Germany overran the Ukraine and took hundreds of thousands to Poland for slave labor. Countless numbers of them died. After the war, the Ukrainian survivors returned home only to have Stalin accuse them of working for the Nazis. He shipped them off to the gulags where the rest of them died.
It's estimated that Stalin killed 20 million Russians during his reign. Both Stalin and Mao made Hitler look like a novice at killing.
