a) Not only allowed by authority figures (Soviet officers) but also heavily promoted by propoganda coming out of Russia via pamphlets saying to show no mercy
b) Done by soldiers who faught for months/years in some of the bloodiest and most zero-sum fighting in the entire war, as the soviets fought their way to Berlin against Germans who were given an obvious suicide mission - to buy as much time as possible for Htiler futile end-game strategies. The Russians also passed many Ukranian/Belorussian towns that were ravaged by Germans.
c) There was basically total anarchy that existed in that area between Russia and Germany. It existed well before the Russians arrived. This stateless anarchy was largely how the Nazis were able to conduct their genocide so effectively and without worrying about punishment.
So this is the same enviornment that the Russians commited their horrible mass rape of German citizens.
China was similarily going through their own anarchistic transition with the battle between communisits (Mao) and nationalists (Chiang Kai-shek) taking place in the power-vacuum. So the pesants who were caught in between existed in a similar brutal enviornment as in eastern europe/Germany and a similar war-ravaged army with a superiority complex operating in an anarchistic state.
Although whether Russia/Japan military were special in their brutality, even in a greater historical context, it's hard to say. There were definately some differences between the Japanese empire/early Soviet Union compared to the west that stand out even today, in terms of harshness of the military towards their opponents/civilians - which may make it stand out.
While many people will say "but the Nazi's were evil too"... the abuse of civilians by Germans was largely kept hidden from their military. The secret police would only come in after the military left an area and it was kept as much a secret from them as possible.
or you think genocide is some 20th century invention?
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
Japanese people are human and the base morality that evolution has endowed us with. This divides humanity into two groups, in group and out group. Morality only applies to the in group. It takes a lot of acculturation and education to expand an individual's conception of the in group is, from family/tribe to nation/culture to humanity. As such, any sub-culture that can take over the public sphere of a culture will have little difficulty getting people to dehumanise the other, and once they're dehumanised people have little more difficulty committing atrocities than they do slaughtering animals.
As you may have noted none of this is specific to Japanese culture. That's because in this respect we're a lot weirder than they are in historical perspective.
If you want a good overview of man's violence towards man and it's decline I recommend Better Angels of Our Nature, Stephen Pinker.