Thread:LucidPigeons/@comment-24469175-20160518184507/@comment-26205772-20160520055522

Give them an influx of character traits and deepen their motivation a little beyond something as simple as "sadism" or "greed." Another good way to flesh them out is to introduce potential redeeming qualities for them or even a good excuse, and then slowly have each quality negated as the story goes on until there's nothing left. A moment of actual, genuine remorse can serve to humanize a Monster however briefly - and them ultimately disregarding and ignoring that epiphany will make that brief flash of dying humanity so much more despicable as a result.

Examples of some Monsters who receive a good bit more characterization than most are Johan Liebert, Amon Go(e)th (from Schindler's List), Koba, Judge Claude Frollo (Hellfire is a treasure trove of character interpretation), Archibald Snatcher (in my opinion, at least), Kilgrave, and Mick Taylor (if you read some of the prequel novels). I suggest studying those characters and see what traits they have beyond just being evil to get an idea of how to flesh out a CM.