I thought it interesting that the most intelligent one, based on IQ tests (which are arguably flawed) was the one who resisted the Nazi regime and was imprisoned for it.

Which to me belies the idea that intelligence is the issue. Intelligence allowed them to create and carry out a horrific plan, but it is not related, in my opinion, to their inherent lack of empathy or compassion or tolerance. Those are the qualities that drove this horror; intelligence was merely a tool that could have be used for either good or evil.