Some of the early myths of Vikings were readily spread by the Swedes. Once they heard tales from traders from mainly Europe they jumped at the chance of being association with such mighty warriors. They regularly spread tales of their people’s bravery all the way over their border with Russia to the east. To this day they still tell tales of it.
Of course with them claiming it was some of their people, at the time their people were keenly aware this was a myth. And findings from excavations of the time have shown written descriptions of how they would get better rates from traders when the threat of the Viking boogeyman was whispered around them. They are a sneaky people and have just happily let the myth of Vikings grow. Over time the legends and myths have become part of their culture, and as many things from the time it can be easy to lose the truth from the stories.