Hitler killed 20 million in Russia alone...
Actually, you will not find two sources which agree on the number of people (Jews or otherwise) killed in the Holocaust or the number of Soviet casualties in WWII. A lot of folks on the wrong side of things took great pains to destroy the evidence.
For instance, according to the surviving German records, fewer than one hundred thousand people died at Auschwitz. No one else kept records of how many people died there, and the evidence from the site is inconclusive-the camp was dismantled after the war and the remaining burial sites were disrupted or lost (the current Auschwitz site was constructed during the 1960s). The commonly reported "2 million" figure is based on railroad records, not actual body counts. Eyewitnesses confirm that a lot of people died, but nobody really knows how many. Russian casualty figures are even more problematic. The Soviets were known to inflate them or minimize them based on the expediency of the moment.
In summary, the Holocaust killed somewhere between 3.5 and 9 million people, depending who is doing the counting, and who they are counting. Likewise, the Soviets appear to have lost about 20 million people between 1935 and 1945, but once again that number is inconclusive and does not differentiate between the casualties of war, famine or purges. There may also be overlap in these numbers, as many of the victims of the Holocaust came from Soviet lands.
I agree with everyone that exact figures cannot be arrived at, if only because people will disagree on which types of deaths to include.
For instance, "Does this count the 2 million Soviet POWs the Nazis murdered from 1941-42?"
Depends what you're counting.
BTW, Stalin killed some large but unknown number of Soviet POWs after their repatriation, perhaps close to this number. The official position of the Soviets during and after the war was that every single Soviet POW was actually a deserter and under a sentence of death.
Oddly enough, a great many of them in the early days of the war probably were more or less deserters. After 24 years under socialism they couldn't believe that the Germans could be worse.
Didn't take them long to find out they were indeed worse, for Slavs at least, but by that time many had already been captured.