What do you mean by human nature changing with Mode of Production? We have quantitative psychological data for a reason, and many of the fundamentals haven't changed since long before the industrial revolution, with many people still ascribing to classical Greek and Roman philosophies or philosophies first found in the Vedic Texts or Dead Sea Scrolls. Clearly then, if these philosophies are still so valuable to modern man, man's psyche cannot have changed significantly, even over such a vast amount of time and even vaster amounts of lifestyle change. I understand that when there is less competition for resources that people tend to be more generous, but that is always true regardless of the mode of production. A surplus is a surplus is a surplus, but when a large portion of the population becomes poor/impoverished, regardless of the state of the elite, that is when tribalistic tendencies most arise. I often see ideas that the elite somehow controls the narrative, which may very well be true today, but was much less so true in the early-mid 20th century.

What we had in Germany was a portion of the population that was predispositioned towards hatred/tribalism/xenophobia/whatever. These are the people that supported and voted for Hitler. Without them, Hitler doesn't have any chance of coming to power. By and large, these people were not bourgeois nor petit-bourgeois, but proletariat, often poor and uneducated. This is the most vulnerable psychological state to the type of culture war that Hitler waged, just as Trump does now, or Netanyahu. These ideas aren't simply implanted into people, but come as a result of hardships. When people are doing poorly, they are more likely to form small groups and turn on the whole. That is a constant throughout history, as it is generally more sustainable to be in a small group, even if it forgoes other luxuries.

To be sure there is often class collaborationism, but to say that is always how fascism arises is ridiculous. For example, Toussaint and Dessalines would return the freed slaves to forced plantation labor. Plantations the generals (and later king for Dessalines) now owned as a result of the war. Another example would be Modhi in India, where there's no real threat of class consciousness because of the historical prevalence of racism/casteism. Similarly, there's no communist threat to Netanyahu's government currently genociding Gazan Palestinians.

If we're taking a more historical definition of fascism rather than my more casual usage that's fine, but I'm gonna need to brush up on specific examples bc this is not entirely my area of expertise.