Nazi seen cruelty, violence and lack of empathy as manliness and virtue. Openly and specifically. You as a young man wanting to prove yourself would be beating and killing people, knowing they are humans and that violence toward humans is what made you manly in your peer group. They were not saying "it does not count". They were saying "I am great for doing it".

> The scope of the ethics is then windowed on who’s deemed human, and who can be slaughtered like an animal for the glory of the great civilization one is part of.

It was not a disagreement about who is human. Nazi did not killed just Jews and foreigners. They killed and tortured plenty of fellow Aryans, because those were their political opponents or to create fear in others. When a nazi tortured Aryan German to get names out of him, he knew full well he is torturing a human. It was not about whether they are human or not, it was simply that human life had less value.

Using animals and insects as insults does not mean there was any confusion about whether those being mistreated are humans.

> Dehumanizing "others" is the classic first step to get rid of any morale/ethical concern when interacting with them.

Actually believing they are not human is super rare and found only in some cults. Insults and degradations are how you work others to a rage, but they are not meant to be factual statements. And they are not interpreted as factual statements.

Happily, nazi left enough writing behind them, we know what they thought about human value.