Not to discourage other answers, but I wrote an answer to a similar question a couple weeks ago, about the show The Last Kingdom

Much of u/PartyMoses' excellent comment applies to The Northman as well, but within a specifically medieval Icelandic context, there's a few bonus things going on. Spoilers below.

The first and most important is that The Northman is by and large adapting and condensing a type of story called a "saga" - that is to say, a prosimetric text (i.e. has both prose and poetry) written in Old Norse between the 12th and 16th centuries, often about the late Viking Age. These are not, therefore, straightforward histories of the Viking World, but literature that both glorifies and condemns Iceland's pagan past (and its contemporary violence in the 12th and 13th centuries). The Northman is highly condensed here - the sagas tend to be long-winded affairs largely concerned with legal cases and the prosecution of disputes. However, these disputes are punctuated with violence in a structure known as a feud (for more on this, see Jesse Byock, Feud in the Icelandic Saga, or William Ian Miller, Bloodtaking and Peacemaking.)

Feuds are, in short, series of reciprocal killings - person from family A kills person from family B, so someone from family B kills someone of "equivalent" worth from family A, repeat until everyone's dead or a settlement is paid. The Northman elides this quite extremely - Amleth's father is so high-status that everyone else in the movie appear to be the price Amleth has set for his vengeance. This isn't really how it works, and Amleth acts far more akin to a monster like Glamr or Grendel (a revenant from Grettis saga or the monster from Beowulf) than a human.

In other aspects, though, the movie really does an exceptional job depicting violence, particularly violence by elite men against marginalized people.

- While I disagree with the Mannerbunde framing of Amleth and his companions early in the film, the amount of violence in a raid is actually pretty plausible, as is the idea of burning people alive in the hall (as happened around 1252 in iceland at the farm in Flugumyri, and probably inspired a scene in Njals saga).

- the perpetual attempted sexual violence against enslaved women is directly on the money. Enslaved women were subjected to constant violence. The human sacrifice at the funeral is also a direct borrowing from Ibn Fadlan's Risala, describing a burial in the Rus', though the ritualized, repeated assault of the drugged enslaved woman in the Arabic text was removed. It's extremely regular, and the only "protection" in the law codes is that only their "owner" could assault them, anyone else was committing a property violation.
There are enslaved women, such as Melkorka in Laxdaela saga, who achieve relative autonomy either for themselves or their kids, but this should highlight only the extent of the violence and its normalization (Melkorka was bought specifically for sexual enslavement), not dismiss the severity of the problem.

-Enslaved men are regularly roped into violence. While I would have expected Amleth's homicide at the ball game to have caused an entire tangential feud, the idea that a random enslaved farmhand would be roped into an extremely violent game and/or straight-up tasked will killing someone else is all over the saga corpus. However, I must emphasize that many enslaved people in the movie do not commit violence, and are instead only repeatedly the victims.

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Once again, these saga narratives are about feuds, they're about killing other people. Are they representative of the Viking Age? No, but also a little bit yes. It's an accurate portrayal of the upper end of Norse dysfunction, when the martial elite of the society align their whole selves towards self-destruction. But it is the upper end, and there are undoubtedly many people who went years, if not decades, without committing or being victims of violence of the sort we see in the film.