My take on villains involves how much they value the in-group over the out-group. A sane person will always balance the in-group good vs. the out-group good. At the extreme end, this develops people who think they are good people because they treat their slaves well.
And then there's just-plain-evil, which actually works in the right environments.
Thank you for this. It had me thinking of Chigurh in NO COUNTRY FOR OLD MEN. A seriously bad guy, but he had his principals, and he adhered religiously to them. Something human we can relate to, despite his dark cruelty. I agree there's something here for protagonists as well. What is the failure, or the loss, or the weakness...something to overcome, something less than heroic?