Why did millions of Germans suddenly decide they were going to follow a "sociopath"? There were deep philosophical roots to the Nazi philosophy that prepared the ground for it. It didn't just come out of nowhere. Antecedents could be found for it in 19th century German philosophers like Kant, Hegel and Nietzche. The heir to this philosophical tradition, the German philosopher Heidegger, became a Nazi when Hitler came along.
The German Marx was also part of this German tradition. The commonality here among all these writers was that they placed the good of the collective over the individual, who could be sacrificed for the good of the collective. The difference between Nazism and Marxism was Nazism was collectivism based on race and Marxism was collectivism based on class. Stalin killed just as many people as Hitler. I would point out, in order to fight the monster Hitler, FDR made an alliance with someone just as bad.
The United States was based on a philosophy, while imperfect, that tried to put more emphasis on individual freedom and individual rights. It was pretty much unique in that belief. Only a few western European countries had similar beliefs and those beliefs never penetrated eastward into Germany or Russia. Putin once said the difference between the U.S. and Russia is that the U.S. puts the individual first over the common good while Russia does the opposite. That is a belief that has always been held in most of the world.