What do you mean by human nature changing with Mode of Production?

Cultural values, ideas, and what's considered Human Nature change alongside Mode of Production. "Humans are naturally greedy" is wrong, Capitalism expresses that trait.

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.

By and large the Proletariat supported the Communists and Socialists, while the Petite Bourgeoisie and Bourgeoisie supported the Nazis.

To be sure there is often class collaborationism, but to say that is always how fascism arises is ridiculous

Far from ridiculous, it's factual. Fascism isn't genetic, and it isn't just an idea thst randomly pops up. Fascism has been studied thoroughly and we know why it exists and where it comes from.

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.

I suggest you do. It's even happening in the US right now, Capitalism is crumbling, and as a result small business owners and corporations are aligning along nationalist lines.