Just to clarify, this is assuming you're referring to the villains that are played 100% straight not the slightest bit of black comedy involved?

Just a tiny piece of advice, I'll say continue from what you've just said and think out what their train of thought is for each action. It helps making the character seem like more of a (bad) person; a lot of times these characters are so awful because of their logic. Yeah most of their CR will end up pretty samey but I'll bet canon isn't any different.