Someone once asked, "What would the barbarians who wrecked Rome think of Nazi atrocities?" I think I have a much broader question from that: what would pre-modern societies think of the Nazis, especially since many of them didn't exactly shy away from what we now call "war crimes."
My view is that many of these societies were products of their time. While we deride the barbarians for destroying Rome, we often forget the Romans weren't especially nice guys. As their empire declined, they also committed grotesque atrocities and betrayals of the barbarians, who often attacked out of vengeance. In short, many societies were products of centuries-long cycles of violence and revenge. Genghis Kahn deserves to be recognized as one of history's monsters, but he was the product of a brutal society of clans, intrigue, and internecine conflict in which cruelty was required to survive.
The premodern world was, to put it mildly, a harsh place. People in the past lacked conveniences that mitigated things like famine, illiteracy, and other social ails. You can see this in pre-modern morality, in which people who fought against one another in war tended to view the evils they committed against each other as regular business.
But the long-19th century saw massive and sweeping changes to the world order that gradually wore away at this kind of mindset: not just technology that made one horrific disease treatable and famine a remote possibility, but new political thoughts that argued for more democracy, more citizen participation in government, and the concept that people deserved rights and liberties. That's not to say the Belle Epoque/Concert of Europe era was all sunshine and lollipops, as atrocities were committed by practically every major country, but the fact that there were movements at this time to condemn such atrocities, such as the international outcry over the Congo Free State's red rubber, Britain's crackdown on the slave trade, and the rise of the NAACP in Jim Crow America was quite a shift from the previous few centuries. There's a reason the speculative fiction of this era was utopian.
In short, people in the pre-modern world would view this era as almost utopian: a poor Roman slave would marvel at the luxuries the European middle class enjoyed in the early 20th century and a woman from medieval Europe would shed a tear over the women's rights movements that preached to her that her opinions mattered.
Then comes along Hitler, a man who completely trashed this social progress in a two-decade-long campaign of terror just to feed an almost insatiable appetite for revenge against an imagined enemy.
The contempt people in pre-modern societies would have for the Nazis would resemble the contempt a refugee to a rich country would feel toward the native-born inhabitants who take it for granted. That refugee may have done terrible things in the past, but they came from a land that had no good options, and once they were given a chance to earn a living legitimately in a new land, they took it, and they mock the spoiled native-born inhabitants for taking for granted the opportunities their wonderful country offers them.
People in pre-modern societies would view Hitler and his acolytes with a tremendous amount of contempt: Hitler came from a world that, while not perfect, was one in which a person could live their life without having to commit evil to survive. And he torched that promise out of sadistic vengeance, turning entire nations into killing fields and corrupting country as an whole into being monstrous exploiters.