Linus is Finnish, maybe this is also a lesson: “Don’t brutalize random neighboring countries because in the future they might be in a position to fuck you in the ass.”?

I will dispute your framing, but why does it justify collective punishment and hatred if all people from a country?

I mean, the Winter War is kind of not a fond memory for them, though everybody loves some Sima Häyhä, one of the most righteous men of the 20th century.

At the time of the Winter War, Finland had existed for about 20 years, same as the USSR. Both emerged out of the Russian Empire. The USSR sought land and space for military defense against its Northern flank near St Petersburg, which was vulnerable, as well as, ideally, ports to seal from water invasions. Finland rejected every attempt at land exchanges, which was of course their right, but the USSR also, correctly, predicted that Finland would facilitate the Nazi advance and that this land was necessary to repel their war. Faced with an existential threat, they invaded Finland and took much of the land they needed and the war unfolded there exactly as predicted, with Finland rapidly becoming Nazi collaborators and putting down most of its internal resistance. The Continuation War followed, of course. To this day, they teach false histories about this, via the usual government censorship and creation of school curricula.

Sima Häyhä was hated by many early on and received many personal death threats to his face. His rehabilitation in pop culture is more of a thing from the 70d and 80s. Finland collaborated with Nazis and built death camps and was subsequently liberated by the USSR. With fascist groups disbanded and banned and the USSR elevated to the status of primary protagonist of winning the war against the Nazis, those who had supported the previous fascist-friendly/just plain fascist government became pretty unpopular for some time.