As vile as the Nazi rule was with 6m murdered there is an even viler system on our planet that has murdered 100m in 100 years it's called Communism. It gets a free pass from the left wing in all Western countries and NOBODY has ever been punished.
Stalin murdered 6-8m Ukrainians in the 1932-33 Holodomor. A deliberate genocide by forcibly taking all the food from vast areas of Ukraine. My wife's family only survived where they kept bees and the bee hives weren't confiscated and they has a cherry orchard. They couldn't risk eating the cherries and so they eat the cherry tree leaves with honey. Many people died in their village along with adjacent cities, towns and villages.
In WWII in a process of extermination by both the Nazis and Soviets in Ukraine there were 1.4m military deaths including, my wife's granddad in 1944 where he served in the Red Army. 7m civilian deaths murdered by both side including 0.5m Jews killed by the Nazis.
I think the difference is Rod that the Soviets were a very important part of the Allied forces that defeated the Nazis. The Soviets were the victors, and so would not be put on trial as the Nazis were at Nuremberg, and infact were a vital element of the prosecution. With Stalin very much in power there was no chance of the West even starting to consider Soviet war crimes. There was no appetite for that, and then of course came the Cold War anyway. Stalin died in 1953, the year I was born, and the Soviet system remained intact under new leaders until the CCCP was dissolved in 1991. There was little chance then of bringing about any justice for the millions murdered by the Soviets during WW2, or later.
Now, in the West, as you have correctly stated, we are in a situation again of having an almost politically closed situation with Putin, who I am sure would not assist with any move to bring war crimes to book. Indeed, as with the Nazis, there are few Soviets alive now who could be held culpable for any crimes. It is not going to happen, and now, regretfully, must be filed in history, but used as a record of what humans can do to each other and as a warning for the future.