Of course Shinzon was crazy - a lifetime of abuse and finding out you're just a faulty, unwanted copy of a great human who lived a privileged life will do that.
Were Hitler's motivations and reasons for wanting to exterminate the Jews any more rational than Shinzon wanting to wipe out humanity and kill Picard to validate his own existence? Hell no. That's how real life crazy works.
They were both shits.
Oh so true.
But to Hitler, his motivations made sense because of a number of reasons (the Jew thing being part of a general anti-semitic sentiment going 'round the world). Smart people could predict basically everything he did based on his 1920s book Mein Kampf, and that was almost a decade before he was elected chancellor. It's really all in the book: anti-semitism, his reasoning as to why the German Empire should expand to the East, the hatred against communism (also widely spread during those days), the resentment against the French....he virtually explained why he did all those things before they even happened. Shinzon wouldn't be able to explain his motivations if his life depended on it.
His life did depend on it, and still his actions made zero sense.
Of course Hitler was a crazy shit, but the enemies were the enemies and he didn't change his mind in the middle of things. If you compare him to Shinzon, it's like Hitler mid-war decided that the Austrians (his country of origin) were the enemy and set off to gas all of them.
So yeah, two genocidal sons of bitches, but not quite the same level of schizophrenic. Real-life villains usually act logically from their own perspective. Hitler was consistent in his insanity. Shinzon is not.