It's hard for me to believe that rational people would accept such blatant nonsense (of course Americans are the most stupid in the world). If I told you that you should join my organization and our goal is to kill all the Jews and killing all the Jews would bring world peace. You honestly would think I'm crazy.
You really should read up more on how the Nazis came to power. Remember, they were elected. And a lot of people who were part of the machinery weren't aware of the full extent of what was happening, and a lot of them were uneasy with it but inclined to follow their superiors' orders anyway (also research the Milgrom experiment). I'd particularly recommend Christopher R. Browning's book Ordinary Men: Reserve Police Battalion 101 and the Final Solution in Poland. Here's an essay I wrote about it in college.
A lot of well-meaning people have been won over by the propaganda of groups like the Nazis and other authoritarian regimes for such reasons.
If I asked you to join the Nazis party, knowing what you know NOW, would you do it.
But that's just the point -- people at the time didn't know what we know now. They were manipulated by lies and propaganda, told what they wanted to hear and guided into compliance. The top people knew what was going on, but they depended on the sheer numbers of rank-and-file soldiers and workers and bureaucrats who were only aware of their own small pieces of the puzzle and blind to the true scope of the atrocities they were participating in.
It's hard for me to believe a liberal diplomat once considered for the Nobel Prize would start yelling Hail HYDRA.
You're simply echoing the disbelief the characters in the episode expressed that Garrett could be HYDRA. It's like Coulson said -- they convince you by being your friend, by hiding their true nature behind a facade of benevolence. The most potent evils are not obvious cackling psychopaths, because then they'd never get into a position of power. The greatest villains are charismatic and charming and likeable. Hitler and Idi Amin were great fun at parties, to all accounts. People liked them and wanted to follow them. They were good at hiding who they truly were.
Remember, HYDRA was playing a long game. They weren't going around doing evil on a daily basis, because that would've blown their cover. They functioned as sleeper agents, playing the role of good guys for years or decades in order to gain rank and influence so they could subtly maneuver things into position for when they were finally ready to make their move.
As for SHIELD's methods, I don't think even SHIELD would even consider murdering 20 million people for the simple crime of thinking for themselves
SHIELD didn't. The HYDRA agents in positions of power within SHIELD did, and lied to the rest about the real purpose of Operation Insight. That's how groups like this get their way -- through lies and deception. What you see is not what you get.