A newly christened society claims the enemy is ungodly, primitive and ruthless. Isn't it always like that? There was probably a good deal of PR already back then.
I think we should be careful trusting the sources on Viking brutality all that much as they are almost exclusively from the Christian side.
The Anglo-Saxon Chronicle is the primary source for documentation of both the Saxon invasion of Roman Britain, and the Viking invasion of Saxon England (about 500 years apart).
The Saxons were very much a similar culture - same use of (very similar) ships for raiding, same weaponry, same battlefield tactics, same civil organisation and even similar languages. The respective invasions were remarkably similar - raiding, mercenary work, followed by progressive occupation. The Viking invasion got complicated - the Danelaw failed and the Saxons beat them off, but then the Normans (Vikings who'd invaded France a couple generations ago) won.
In the Chronicle the Saxon invasion is portrayed very much as a "we were invited here!". The Viking invasion as a brutal series of raids.
No such thing as bad press I guess - Vikings are cool and Saxons are a bit uncool now ;)
Why would Christians have lied about the ferocity of their invaders knowing that they actually ravaged and pilled the whole of Western Europe later on? If anything History has proven they were right.