Most people are aware of the cruelty of the SS towards Jews and others they regarded as inferior to the Master Race. What my WWII vet father told me is that to join the SS an applicant had to prove a documented act of cruelty against the German people as well.
At the end of the war my dad's outfit was processing an entire surrendered German army. They'd have each soldier roll up their sleeves to check for a distinctive tattoo that most SS had and if so they were separated out from the rest for special handling.
This might be where J.K. Rowling got her idea of the Death Eaters having a special "Dark Mark."
Evil is often rooted in jealousy. Knowing the depths of their own inability and cowardice, men turn to hatred to excuse their sin. Once compromised, these weak men conspire to ensnare others, rather than accepting the consequences of their behavior.
During WWII, Americans had the moral conviction to guide them through uncertain times, but I fear we as a nation have lost that foundation. I have often said that there could be no worse evil than an America without moral restraint, and we are close to that precipice.