When you look at the origins and histories of these royal families, you realize that they basically are descended from a bunch of warlords essentially like Mafia godfathers or Arab sheikhs. Murderous, backstabbing, conspiratorial trash. They lived off of taxes on the lower orders and spent their time riding to the hounds, screwing harems of mistresses, and fighting other nobles for power.
It’s good to be king. :-))
To your point, the first Duke of Normandy was Rollo, a Viking warrior believed to be a nobleman from Denmark or Norway. He raided Northern France and up the Seine until the King gave him Normandy if he would quit raiding and swear fealty. His descendant, William the Conqueror, invaded England and seized the Crown. The rest, as they say, is history.