Why were the Vikings viewed as the hostile outsiders in medieval Europe
The Viking Age, that is, the period in which Vikings were routinely venturing into England and continental Europe, begins roughly in 700 A. D. and ends about 1000 A. D. when Vikings had settled in various parts of England and the continent, especially England's south and east midlands, a part of France we now call Normandy (named after the north men), and large parts of what is now Russia and, of course, Scandinavia.
Because the Viking expansion began in such a spectacularly violent way in several European countries, especially England, the general view of Vikings during the Middle Ages is almost exclusively negative. Beginning in 793 A. D., for example, when Vikings raided and destroyed one of England's holiest sites, Lindisfarne Abbey, Vikings became synonymous with almost mindless violence, with no goals other than to take portable property and people in order to sell as slaves. The entry of the Anglo-Saxon Chronicle for 793 indicates that
A great famine followed and a little after that, on the 8th of June, the ravages of heathen men miserably destroyed God's church at Lindisfarne.
Vikings, often called "wolfcoats and berserks," became, through their incursions into France (all the way to Paris), England, and Russia, major traders in slaves. At Armagh in Ireland, during a raid in 869, the Vikings took almost 1,000 men, women, and children were taken and sold into slavery. Because they had no use for real estate, they destroyed holy sites, towns, and villages everywhere they landed.
In England alone, the list of monasteries devastated by Vikings--Iona (Scotland), Jarrow, Durham, Mondwearmouth, and Whitby--covers the major repositories of learning in England at that time. In an age when education was restricted to very select groups, for the most part, the clergy, the destruction of monasteries included the loss of an immeasurable amount of literature and history, much of which has never been recovered. We know about the existence of many manuscripts only because they are referred to in surviving manuscripts. There is actually no way to measure what the Vikings destroyed.
In the 800s, Vikings began their incursions into France all the way to Paris, and even though Charlemagne built forts along the River Seine to stop the Vikings, they managed to overcome these obstacles and threaten Paris time and again. The cost of defending France became so expensive that French kings began to pay tribute to the Vikings to stay away, a technique used in England (with as little success). Even so, by 870 A. D., the Vikings had established a colony in what is now Normandy, and shortly thereafter they were given that province to control.
By the 11thC., partly because Scandinavia had stabilized politically, Vikings (actually, their descendants) had colonized about half of what is now modern England (eventually, the area was called the Danelaw), and because stable and, by this time, Christian rulers were established in Scandinavia, the forces that compelled the earlier raiding behavior began to decrease. Stability within what is now Scandinavia, as well as permanent colonies in parts of England, Ireland, Russia, and France, replaced the raiding culture that characterized the first 250-300 years of the Viking-European experience.
Unfortunately, the violence--which was both physical and spiritual--with which the Vikings began their history in Europe forever stigmatized the Vikings as destroyers of culture. The people of the Middle Ages were still too close to the violence and devastation of the Viking expansion period to view Vikings as anything other than "wolfcoats and berserks."
Vikings were basically raiders. They would sail up to the shore near a village and attack and kill the people. They would leave with all of their belongings, along with some of the children. The Vikings were fierce and mysterious, which is why they were looked at as hostile outsiders. "Vikingr" in Scandanavian can be roughly translated to "pirate."
The Vikings were essentially Pagan Scandinavian Normans who made it their job to terrorise much of Europe from the 9th to 11th century. They were fierce and enthusiastic warriors that raided the coastal cities and towns of Europe, burning and looting their way through the continent. Their pagan beliefs would have also stood in stark contrast against the religion of Christianity practiced and adopted by most medieval European states. Viewed as an unwelcome presence, the Papacy had attempted to rein in the Vikings by creating a papal army to drive them out. The military was however annihilated and the Church was ironically forced to ally themselves with the barbarians, having failed to eject them from Europe by force. The Vikings were viewed with such hostility that the kings of France often paid them off to serve as mercenaries, settling them along the northwestern coast to defend it from raids carried out by other Vikings.