I’m gonna tackle a particular piece of this, and Hank can jump in and steer things his own way too.
The best “evil” characters, IMHO, are often the ones who don’t know they are bad, who truly believe in the (usually tragically wrong) cause they are supporting. This allows the writer to create a more complex villain – see Disney’s retelling of the Sleeping beauty tale in Maleficent, a character who did terrible things but had her reasons.
OK, so she was mostly justified. But you get my gist.
So how do you handle “evil” characters? And what are some of your favorite nuanced bad guys (and girls), and why?