Introduction: Here's a misconception about World War II that I think is harmful and I don't see refuted often enough.
Misconception: In 1941, Hitler was sitting pretty with most of Europe conquered and no huge difficulties on the horizon. Then, due to his megalomania and bullshit ideology, he decided to invade Russia. This was an unforced error of epic proportions. It proved his undoing, like that of Napoleon before him.
Rebuttal: In hindsight, we think of the Soviet Union as a superpower and military juggernaut which you'd be stupid to go up against. But this is not how things looked to the Germans in 1941. Consider World War I. In 1917–1918, Germany and Austria had defeated Russia at the same time as they were fighting a horrifyingly bloody war with France and Britain - and another devastating European war with Italy. In 1941, Italy was an ally, France had been subdued and Britain wasn't in much of a position to exert its strength. Seemingly, the Germans had much more favorable conditions than in the previous round. And they won the previous round.
In addition, the Germans were not crazy to think that the Red Army was a bit of a joke. The Russians had had their asses handed to them by Poland in 1920 and in 1939–1940 it had taken the Russians three months and a ridiculous number of casualties to conquer a small slice of Finland.
Nevertheless, Russia did have a lot of manpower and a lot of equipment (indeed, far more than the Germans had thought) and was a potential threat. The Molotov-Ribbentrop pact was obviously cynical and the Germans were not crazy to think that they would eventually have to fight the Russians. Being the first to attack seemed like a good idea and 1941 seemed like a good time to do it. The potential gains were very considerable. Launching the invasion was a rational military decision.
Why this matters: The idea that Hitler made his most fatal decision for irrational reasons feeds into the conception that evil and irrationality must go hand in hand. It's the same kind of thinking that makes people think a superintelligence would automatically be benign. But there is no fundamental law of the universe which prevents a bad guy from conquering the world. Hitler lost his war with Russia for perfectly mundane and contingent reasons like, “the communists had been surprisingly effective at industrialization.”