Norway is a mountainous country with deep valleys and fjords carved by glaciers. This geography of isolated valleys meant every valley was a kingdom or a tribe which means a lot of fighting. Denmark and Sweden are mostly flat.
The terrain was harsh and sunlight was scarce. Long nights in the winter and bitter cold. The main foods were fish and reindeer. If you ran out of food the choice would be starve or raid a neighboring village. Violence would have a distinct survival advantage. I don’t know why the people who write this stuff are so surprised by it.