What made the Germans behave as they did during the war?

It is because they are people and people can be like that if they get uncontrolled power.

It can happen (and has happened) again.

Just a few examples...

The Roman Empire
Slavery in the South
Uganda under Idi Amin
Pol Pot
Iraq against the Kurds
Iran after the Islamic Revolution
China in the Cultural Revolution

And many others that I've overlooked.

You can see it starting every day in your own country when petty bureaucrats get a little more power than they can handle. It starts small and then gets out of hand if it is allowed to continue.