and were particularly hard on monasteries was because of edicts being handed down that forbade Christians from trading with non Christians. While that was aimed mostly at the infidel Muslims in the Middle east who had taken control of "their" holy land, Vikings also felt a chill from that policy. Once they found out how much bling those impoverished monks had stashed, the race was on.
Rome and Byzantium had wanted to keep the profits in the family during the Crusades. Vikings objected,
They were definitely a mixed bag, preferring to trade if there were enough men around to defend a village, taking captives for ransom if there weren't, selling those captives elsewhere if they weren't ransomed, and being ruthless fighters if anyone gave them much of an argument. They had huge trading networks and founded settlements wherever they went. Eventually they threw in the towel and slowly became Christianized, trading without raiding.
The Vikings weren't all that illiterate, histories were kept in the colonies of Iceland and Greenland. The bulk of the sagas were written down 1200-1500. Some were pre Christian, some were transitional and if you've never read them, consider it, it's great stuff full of love and jealousy and blood feuds and magic swords, as well as the more mundane details of who came when and who married whom.