Of course, atheists believe in not believing in God, so they take events like the Holocaust as proof that God doesn't exist. I choose to think of events like that as proof of how much men need God.
Anyway, K T Cat suggests that people should have seen it coming in regards to Hitler:
Hitler wrote Mein Kampf long before he came to power. Anyone who read it knew he was trouble from the get-go for Jews in particular and Europe in general. Never one for subtleties, while he doesn't lay out the blueprints for the gas chambers, it's hard to see how it all ends well for the Jews. When he finally did seize power in Germany, the sequence of events, including Kristallnacht, made it clear to everyone where this was headed.People can't be bothered to learn from history. And it's not just those that are doomed to repeat it. They're going to drag the rest of us with them. Unless we act.