“..The Nazis did not start with rounding up and killing people by the millions....”
No, you’re correct; they didn’t.
First, they passed gun control/confiscation - including the “Sporting Purpose” clause (read the democrats Gun Control Act of 1968, which they copied from the Germans).
And Germans, being good “law-abiding citizens”, complied. THEN they did the rounding up.
Because nobody could shoot back.
And they used the old lists of firearms owners that the Kaiser’s people generated in the prior “war emergency” during WW1 that were sitting in file cabinets for 20+ years.
The Nazis took a trajectory of escalation. They took away guns, yes, but they also demonized and ostracized people. Then they started destroying their homes and properties and harassing and intimidating them. They had to work themselves up to the point where they had successfully dehumanized and demonized certain groups of people before they started imprisoning and killing them.
It seems that every socialist regime has a group of people that it deems enemies of the state in order to justify killing them. I suppose that if they can keep their loyal followers focused on the “other” as the source of societal problems, it keeps the followers from perceiving that the socialist dictators are the true problem.