The Vikings were unusually bloodthirsty and barbarian
The Viking raids were indeed very violent, but it was a violent age, and the question is whether non-Viking armies were any less bloodthirsty and barbarian; for instance, Charlemagne, who was the Vikings contemporary, virtually exterminated the whole people of Avars. At Verden, he ordered the beheading of 4,500 Saxons.
What really made the Vikings different was the fact that they seemed to take special care to destroy items of religious value (Christian monasteries and holy sites) and kill churchmen, which earned them quite a bit of hatred in a highly religious time. The Vikings probably enjoyed the reputation they had; people were so scared of them that they often fled from their cities instead of defending them when they saw a Viking ship coming near.