Danelaw, Visigoths, Ostrogoths, Vandals, Lombards were all vicious and cruel conquererors. They had no mercy on their subjects. It was, "Do as I say, or I'm gonna rape your wife." They were oppressive and exploitative too. Some drove off the original peoples of the area in a fashion similar to the Americans driving off Natives. I'm glad the Moors gave them a taste of their own medicine.
Almost every Nation/kingdom power that has existed has committed acts that today we would think of as illegal or unthinkable, if they didn't they would have likely been conquered by their neighbors.
A kings power back then relied apon his ability to protect and provide for his people, if the king could not do that he would be deposed or his people would be conquered for his failings.
The so called civilized powers commit-ed their fair share of crimes and atrocities, you think every country has wanted to join the various empires that have risen throughout history? The strong have always ignored the weak when they find it convenient to do so.
Without the aggressive expansion of the Germanic Kingdoms, the borders of Europe would be considerably different today, the various conquers replaced many small kingdoms that sprung up as the Western Roman Empire collapsed. Laying the ground work for the rise of strong nation states most of which have survived to this day.