Historically war basically consisted of everyone getting together in a big field and having a large fight. People who lived all of half a mile away could just get on with their lives as long as they didn't go near the war field.
It’s something that happens when you give people weapons and let them loose in another country knowing there is little oversight or consequences. Just allows the bad guys to finally do what they want. The “regular people” contain an underestimated amount of assholes.
Russia has always used state sponsored terror tactics and humanities violations. Just go read up on the way they treated people in their way to Berlin in WW2. It's horrifying. Raping children to death in front of their families is just standard operating procedure for the Russian army and that never stopped being true.
Now, they've used another time honored Russian strategy. They emptied the prisons/gulags to flesh out their front line. Which just feeds into the depravity of the troops who are well known for this kind of behavior already.
It's normal for war situations. The kind of people who become soldiers are almost always open for raping other people when given the chance. If you read about other wars, rape is always very common.
They are programmed into believing that the other country is evil, so that justifies their actions in their own minds.