Hitler was most certainly (according to every respected historian/Nazi studier) NOT insane.
I read it many times that he was paranoid.
To call him insane dismisses him as a one-off and lulls simple people into thinking it can't happen again
No, what makes you so? Paranoid or not, he captivated Germans with his charisma and made them believe they would be great again., The slogan Make America Great Again was invented in Germany first. It was the enthralled nation who voted for Hitler.
So, preventing the horror from happening again is possible by educating the masses so that they won`t believe maniacal leaders who promise them greatness.