The sources we have on viking raids are either, as /u/vhite said, from the victims or from 13th century or later Icelandic accounts. That said, I'm not familiar with any víkingar who are mentioned as butchering entire villages and towns. The closest I can think of is Arinbjörn and Egils raid on the Frisians, when they sent the whole village into flight and chased after them. Even then, there's no mention of wholesale slaughter.

During the Danish invasion of Britain, that might have happened with the Great Heathen Army, but they were neither víkingar nor raiders, and so don't really fall under your question.

I can't answer this question but it helps to think that Venn diagram of people raided by vikings and literate people of northern Europe would have pretty large intersection. So it's likely that sources on vikings may be little bit exaggerated if taken literally.