Scandinavia at the time (and a few centuries after) was a violent clan society. War, raids etc were what people did instead of playing football and ice hockey. And business.

The trips to (e.g.) England were business, it is documented that boats were recruited from all over Scandinavia.

The "real" Europe at the time had [to a large part] gone on beyond that stage [of clan societies].

What to learn from this is not to be neighbour with violent clan societies. Think old Scandinavia or in the present day -- Afghanistan, Somalia and eastern Iraq.

(Incidentally, Sweden has the world's largest refugee immigration per capita from exactly those places... We will see how that ends up.)

Edit: I guess the down votes are my parallel with modern clan societies? Time will tell, I hope I'm wrong.