I haven't seen the show, but it sounds like it might be overstating the barbarism of the Scandinavians in order to give the monk the modern moral high ground. There are accounts of some people having up to thirty thralls, (the slave-equivalent the show must be talking about), but most households who had thralls at all had one or two, so killing one would have been a bad economic loss. The plantation economy with large work-gangs of slaves that has shaped the US view of slavery was not the case for Scandinavia.

Moreover, thralls could be freed by their owners or even buy their own freedom, and freed slaves were apparently frequent enough to be seen as their own social class with rights legally regulated.

Now, in Roman Britain slavery had been common, and the Anglo-Saxons (culturally very similar to the Scandinavian "vikings") merget this into their own practices. So while "chattel slavery" was phased out after the Norman conquest of 1066, the Domesday book census of 1086 lists 10% of England's population as slaves. In addition, I think it can be argued that the indentured serfs common in Britain and continental Europe into the Middle Ages were not that different from thralls.

For more on thralls and Scandinavian slave trade, see for instance this National Geographic article, and for social structures in viking-age Scandinavia see Larsen 2012, or for the legal status of thralls Ruiter 2013 (unpub.). There is a decent litterature on English serfdom and slavery, for instance Bush 1996

Slavery was a common phenomenon in Anglo-Saxon England, although the precise scale of it is slightly tricky to pin down. In Domesday Book, roughly 10% of listed households are slaves, although it's not clear if a slave 'household' was the same as other listed households, with approximately 4 people, or represented simply individual slaves. That would distinguish whether slaves were seen as a distinct social class, or as a collection of individuals.

Slavery is referenced extensively in English laws from the mid-7th century onwards, and slavery laws seem to evolve with a growing sense of English pan-nationalism. In the early Kentish laws of the early 7th century, burglars caught in the act could be sold into slavery overseas. By the reign of Ine in the late 7th and early 8th centuries, the legal code of Wessex expressly forbid the selling of any Englishman into overseas slavery, and also introduced some basic legal protection for slaves.

By the reign of Alfred of Wessex in the second half of the ninth century, slavery is increasingly codified. By this period, the vast majority of slaves are likely to have been Welsh or Irish. By the early 10th century, Irish slaves are an important commodity being imported into the major Mercian port city of Chester from the Hiberno-Norse settlements at Dublin and Waterford. Under West Saxon and later English law, slaves are given some protection; slaves are part of the weregild system, albeit at a lesser value than any freeborn English, and are not allowed to be ordered to work on Sundays, among other rules and protections.

An English monk is therefore very unlikely to be surprised by slavery, and indeed it's not unlikely that a monastery might in fact have slave households within its own holdings. He might, potentially, be surprised by the savagery of Viking slave raids, but on the other hand, Vikings takes a great series of extreme liberties with the truth on a regular basis.

Slaves were common during Clovis' reign in Francia just a few centuries earlier and is written into law books at the time (such as penalties for killing another's slave). Without doing specific research myself, I would think this had not changed much by the 8th century. Survivors of a defeated army would most likely have become slaves.