[brobdingnagians]

An interesting side note on this, the descendants of the Vikings were also the Norman's, that later invaded England, and even sailed around to Southern Italy and conquered a place down there as well (I think it was somewhere roundabout Sicily?). They developed the state of the art castle building techniques, sailing techniques, extensive trade networks, and even some interesting governing techniques (women had land-owning rights) that enabled them to conquer and hold much of Europe over time. It might be that they were "the most terrifying raiders of the sea" because they had cultural values that prized technological advancement and decently good governance, and then spread out to nicer climates (good motivation to try something new...). Interaction and adoption of values from other cultures probably helped bring down the violence after a certain point. And like someone else pointed out, when you get wealthy and own the kingdom, you typically want to develop a culture of preservation rather than raiding.