But that still doesn't explain why the armies of Western nations seemingly committed less and less serious atrocities. Or is that just an illusion because "history is written by the victors"?
I'm guessing it's because Germans had racial propaganda dehumanizing Eastern Europeans (Slavs are "untermenschen") and the Russians retaliated once they were able to. They had seen far more gruesome behaviour by the Germans and had less inhibitions to treat them in kind. The Americans and British, OTOH, had merely been exposed to the far more "conventional" horrors of war (i.e. civilian casualties from war and occupation) until they found out about the camps.
As for the lack of atrocities committed by the West: don't for a second believe anything about the Pacific theatre of WW2 was lacking atrocities. Japan quite literally raped China, yes, but the US routinely firebombed Japan until they literally nuked city centres.
The fire bombing of Dresden (a German population centre) is considered a morally questionable decision these days. The routine fire bombing of Japanese cities is not even talked about. And whether you consider fire bombing civilians a war crime or not, the nuclear attacks on Hiroshima and Nagasaki most definitely were.
The thing to know about war crimes is that the legal framework for defining and punishing war crimes we have today (even if we make far less use of it than we should) didn't exist back then. There was no legal basis for the Nuremberg trials, legally speaking, other than victor's justice.
The reason no Allied (including Soviet) commanders or politicians were tried after WW2 wasn't that they didn't commit any atrocities, there were simply no international laws they could be charged for violating.
Note: I'm not arguing whether the way the US fought Japan worked or not, or whether the death count would have been higher or lower had they done it differently. I'm merely arguing that objectively speaking not only the "bad guys" have done horrible things to humans in modern wars. And I haven't even mentioned how the Vietnam war is perceived outside of the US.
The Russians also distributed plenty of propoganda pamphlets to their troops to show no mercy to the Germans (including citizens) as they faught their way towards Berlin. Propoganda comping from some high-ranking soviets, not just within the military. It was said to be a big contributing factor to their notorious behaviour afterwards.
Humans who are taught to dehumanize others are capable of doing pretty horrible things.
History is written, there's just more eye-catchy things to get distracted over.
Game of Thrones. Brexit. Reality TV. Concerts. Drugs. Sex. Food. Cars. Video Games.
On a road trip, a Serbian in-law had us stop by this beautiful bridge in Bosnia. Built in the 1500s with Ottoman architecture, which is odd for that area. I took photos there, smiling and having fun like an idiot. Only after leaving and checking wikipedia on my phone did I see that the relative failed to tell me about the massacres that happened there by the Serbs. Women, children, rape, etc.
Wikipedia article: Visegrad Massacres