As far as I know, Robespierre, Lenin, Stalin, Mao, and Pol Pot were indeed unusually incorruptible, and I do hate them for this trait.
Why? Because when your goal is mass murder, corruption saves lives. Corruption leads you to take the easy way out, to compromise, to go along to get along. Corruption isn't a poison that makes everything worse. It's a diluting agent like water. Corruption makes good policies less good, and evil policies less evil.
I've read thousands of pages about Hitler. I can't recall the slightest hint of "corruption" on his record. Like Robespierre, Lenin, Stalin, Mao, and Pol Pot, Hitler was a sincerely murderous fanatic. The same goes for many of history's leading villains - see Eric Hoffer's classic The True Believer. Sincerity is so overrated. If only these self-righteous monsters had been corrupt hypocrites, millions of their victims could have bargained and bribed their way out of hell.
-- Bryan Caplan
Hitler was at least a hypocrite - he got his Jewish friends to safety, and accepted same-sex relationships in himself and people he didn't want to kill yet. The kind of corruption Caplan is pointing at is a willingness to compromise with anyone who makes offers, not any kind of ignoring your principles. And Nazis were definitely against that - see the Duke in Jud Süß.
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