Without seeing it, I'm wondering if the film is based on the book
Savage Continent: Europe in the Aftermath of World War II by Keith Lowe. I may need to move that closer to the top of my reading list.
One thing that is largely left out of the history books (especially at the high school and even college World/European history survey level courses) is the amount of what we now call ethnic cleansing that went on in the aftermath of the war. While this mostly involved pushing ethnic Germans out of various areas, including most of what was once Prussia, it also involved Poles being pushed out of western Belorus and Ukraine and into old Prussia (somewhere I have a German map that shows the regions of Prussia, Pomerania, and Silesia as "Unter Polnisher Verwaltung" - Under Polish Administration.) Areas of Finland that were taken by the Soviets were sparsely populated, but I'd bet that most ethnic Finns living there moved out as well.
Ethnic cleansing is a bad thing, and almost always, as noted accompanied by rape, murder, and all sorts of terror and awfulness. This process did result in much more ethnically/nationally homogenous populations on both sides of the various borders. (Especially areas bordering the new Germany. One of the unstated - or at least left out of most history books - objectives of the war was to destroy 'Prussianism' once and for all.) And the effected areas have been largely free of conflict in the 70+ years since (although the Cold War had much to do with that.)
The one place that remained heavily mixed - Yugoslavia - suffered extremely nasty fighting once the Cold War lid (and Tito's dictatorship) were removed. And other areas with ethnically mixed populations - eastern Ukraine, Moldova - remain flashpoints and/or low to mid-intensity war zones. (I will note that most, if not all, of those such areas east of Vistula River involve populations of ethnic Russians whose ancestors were mostly planted there under Stalin.)
It's open to debate how much of the barbaric behavior exhibited by the Red Army once they entered German territory was payback for the equally barbaric behavior of the Germans on Soviet soil (Einsatzgruppen, anyone?). Not that it excuses said behavior, but it does much to explain it.
I will note that the Soviets' treatment of their own soldiers who had been captured by the Germans (and somehow survived to be repatriated) was truly brutal - but I think that had more to do with Stalinism/Communism than anything uniquely Russian.