I don't think the vikings were uncommonly violent. Yes they had a culture with lots of warrior kingdoms but that's not unique to them. One of the reasons that made them such savages in the eyes of history is simply their difference compared to the mostly Christian majority of Europe at the time. Two heathen things the vikings practiced were (human) sacrifices and the looting of Christian religious sites. Both were considered acts of pure evil to the Christians. For the vikings they were just normal parts of life. As was their difference in the definition of murder and monogamy as well as homosexuality. If I recall these things correctly murder was killing a man without giving him the option to defend himself and not report the death right away. It was above all seen cowardly to run from a killing. And they really had issues with cowards. Homosexuality is the same, fucking a man was okay but being fucked was weak. Only the receiving party could be punished I believe. As for monogamy, they had marriage but there was far less moral damning for sleeping with another. You see how these things seem barbaric to a devout Christian? It simply gave them a bad name.

I haven't seen the show Mankind at all so I can't reply indepth to their arguments but in my understanding the crusades started mostly as a political power struggle between the Catholic church and the Fuedal lords and kings. Wars like the crusades are extremely expensive and forcing lords to spent money on it meant the church had relatively more then them. It also took care of the bored workless soldiers they had around between wars. THe fact that the common man joined in with peasant armies was just a by-product and I don't think the church needed or wanted them beyond encouraging faith.

What other acts are you referring to?

While I haven’t seen the show in question, this is almost certainly a reference to the Normans. These were the descendents of ‘Norsemen’ who settled in northern France (what is now Normandy) in the 10th C. Referring to them as ‘Vikings’ is something of a stretch given that they quickly acclimatised to their new lands, adopting Frankish language, customs and ways of war. In the High Middle Ages they were at the forefront of Frankish expansion – campaigning in England, Aragon, Italy and the Levant. In terms of the latter, two of the contingents of crusaders during the First Crusade were Norman, one of which, Bohemond of Taranto, managed to carve out his own fiefdom at Antioch.

But while they were clearly a violent bunch, were the Normans that much more violent than their neighbours? Orderic Vitalis believed so – memorably noting that their “bold roughness proved as deadly to their softer neighbours as the bitter wind to young flowers” – but I’m not convinced. This was an exceptionally violent era. The Normans were good warriors and could be brutal in their subduing of territory but I’ve not seen anything to suggest that, say, Robert Guiscard was notably more violent than a Fulk Nerra or even a El Cid.

The First Crusade actually bears this out (somewhat). Robert of Normandy was there from the start (Bohemond’s contingent being picked up as they passed through Italy) but most of the key ‘princes’ were non-Norman. To reel off a few names, the remaining major figures consisted of Godfrey of Bouillon, Baldwin of Boulogne, Raymond of Toulouse, Stephen of Blois, Robert of Flanders and Hugh of Vermandois. This is a fairly representative geographic spread of the Kingdom of France at the time – I don’t see any reason (beyond the poetic) to single the Normans out or suggest that they were a cause.

And that’s as far as I’d feel comfortable commenting without seeing either the specific claims from the show or launching into a much wider answer on the nature of medieval warfare. Hopefully that’s not too narrow a response for you.

Sources: Most of my reading on the Normans relates to Italy but Marjorie Chibnal’s The Normans is probably as good an introductory work as any on the subject. The First Crusade paragraph is pretty basic but I’m happy to recommend reading if required. See also Robert Bartlett’s Making of Europe for a perspective of the role of Normans in the broader wave of Frankish expansionism during the period.