It is hard not to associate "viking" with rape, slaughter, and horrific treatment of captives and defenceless peasants. And yes, by definition a viking made a living through plundering, which is inevitably a violent business, where genuine psychopaths find an environment they can really cut loose in. But were the Norsemen actually any more harsh than other peoples, during this chaotic, war-torn period? Did the disunity and tough living conditions of their homelands breed a really unpleasant culture, or did Christians just demonise them due to the whole "robbing churches" thing?
My three(!) post long response to a very similar question answers this in pretty good (I think) detail. See here
I will expand on it very slightly to push back a little on the term "propaganda." Your point that Viking activity (and consequently Norsemen) are demonized "due to the whole "robbing churches" thing" is not wrong but its worth thinking about the fact that they did rob, kill, and kidnap a lot of people, including churches and that that trauma must have been very real for many of our Christian authors.
This is not to say that Christian authors don't exagerate or even use the Viking threat for their own purposes. Hincmar of Rheims, one of the most famous and prolific authors of the 9th century, for instance, notoriously turned a Carolingian victory at Saucaourt into a defeat in the Annals of Saint-Bertin because he wanted to be pissy about the state of Carolingian rule. So we certainly should be reading our authors with a grain (or heaping tablespoon) of salt.
But when authors like Alcuin dehumanise the raiders of Lindisfarne its not just because they want to "demonize" their opponents, it is also a completely understandable reaction to a trauma inflicted by outsiders. Does this make for good accurate history reporting? No, and our job is to unpack and account for (and even study) that trauma. But it doesn't mean we should dismiss accounts of Viking violence out of hand. Understanding why people wrote about the Vikings in these ways is as much a historians job as understanding how and why Vikings acted in the ways they did.