I think it's because you presented a very simplistic point.

Any person with a little history knowledge would indeed be able to say that what you said is correct. Any person with a little more history knowledge will understand that the reasons for WWII were complex, and the more you try to generalize from Hitler and his circle of friends to the entire population of Germany, the less clear it makes sense.

If WWII teaches us anything, it's that everyone is vulnerable to the same forces that created the Reich. It also teaches us that the more hate and more division isn't going to help anyone. This is what I fear "builds up the ingredients for the next downfall of humanity" - more hate, more outrage, more division between people. More politics and status games.