[andrepd]

Why did Germans go from having a functioning liberal democratic society, to becoming some of the most cruel barbarians in human history, to becoming a peace-loving nation, all over the course of a few decades?

Things are complicated and the forces of history move in unpredictable and difficult to explain ways. We can identify broad trends which change cultures and societies: catastrophic events, economic changes, political changes, etc. But there's often no single explanation.