The Swedes were on expeditions to the east and as far as Constantinople back in the day. They were all up and down the Volga and through area now called Belarus too. Before Constantinople fell this was very much a peaceful trading venture.
They left much of their genes there. See Byelorussia too. (Belarus)
From the other direction (central Asia to west) came The Huns !
http://www.ancient.eu/Huns/
Swedes were active in asserting themselves in Poland and the Baltic states later too.
https://en.wikipedia.org/wiki/Polish%E2%80%93Swedish_War_%281626%E2%80%9329%29
The brutality has origins in the Mongol Hordes
https://en.wikipedia.org/wiki/Mongol_invasion_of_Rus%27
The Russians got the message eventually: They would have to get mean and powerful to protect themselves.
Still a struggle much later with Napoleon and Hitler. Even success is depleting when there has been great struggle.
The Battle of Stalingrad ranks with the most major battles of all history, maybe exceeds most all.
Russians and Germans poured men, weapons and supplies into this huge meatgrinder and it was a total waste, even though Russia was victorious. You know what I mean? All remaining can hardly stand up from the exhaustion, all is rubble, frozen dead bodies everywhere, and wholly inadequate food and clothing?
Do you really know? You should check it all out very carefully.
https://www.youtube.com/watch?v=61eYOpEF7hk
The Germans were very, very BRUTAL under Hitler, and the Russians became likewise, thus only about 600 of Von Paulus' Stalingrad troops returned home eventually after WWII (remainder of 90,000 killed as far as I know, non-detail historian I am, so could be wrong somewhat; battle killed 1000 Germans/day quite often).
You really have to see the videos for long time, no description would appear true elsewise, the battle was awful beyond belief.
It will help you to understand how Russians became as they are, all of this answer.