Yes, I gather that Stalin was the iron fist type, rising though pure ruthlessness,

That gives him too much credit. Stalin was relatively inconsequential in the rise and initial rule of the Bolsheviks, which happened under Lenin with Trotsky as a strong second in command. He just managed to palace intrigue his way to the top after Lenin died.

He did keep his power by purging everyone all the time, which is another thing that seems to work better on humans than pure rationality would suggest it should. People were never risk-tolerant enough to stop him, but also never risk-averse enough to avoid working for him, probably out of hubris.

I’m currently working with a German guy, and am trying to.figure out how to broach the subject of how did common, decent people become so indoctrinated to an extreme right ideology.

I'm going to recommend Ordinary Men, which is a book cataloging and analysing accounts of members of one of the battalions responsible for machine gunning people into ditches.

A random German won't necessarily be super into history. A random Israeli is liable to say it had nothing to do with fascism and everyone is always against the Jews specifically. History is still going on there, and there's no objectivity.