Writing the "beast" is much easier if you're a fantasy author. Within that framework you can build all kinds of beasts and make them good or evil or a combination of both. But when you're constructing general fiction, the beast lives within a person and his or her actions determine just how much of that innate evil is allowed to live and thrive.
I've had the opportunity to meet author Steven James when he taught some classes at a large, multi-faceted church function in my area. I invited a friend to help me "man" his book booth while he spoke and taught classes. If you're unfamiliar with Steven James' novels, you need to know he writes some of the best contemporary thrillers in literature. A Christian author with a Masters in Storytelling, he selectively teaches writing classes, and he rails against "Christian" being inserted in front of fiction.
A tall, slender, nice-looking guy who sometimes wears hip glasses, Steven James is soft-spoken but enthusiastic, athletic, nice, and gives the impression he could be a lot of fun in a less professional setting. Steven James writes some of the most evil characters you will ever discover. Horrible. Creepy. Yet often appear to be ordinary, suave, or accomplished. He has stated he wants to capture evil and demonstrate its emotional content. The question you might want to ask is, "How does a nice guy like you write such despicable characters?"
To write evil you do have to understand its source. The most painful part of writing it is knowing if you travel down deep enough, you can find it inside yourself. Not a good feeling. But true. Granted, not all of us can go that deep and come up with the stuff that fills the pages of thrillers, nor can all of us afford to examine evil to the length required to write the kind of mindsets which comprise the makeup of serial killers. That's a good thing. It's more than some can bear, dangerous to look at or study, and completely not suited to the fearful and/or delicate psyches.
We are to hate what is evil, but if we make evil "typical", it makes for boring reading. Evil comes in so many forms in human conduct, but it inevitably begins in the mind. Creating an introduction to the beast hidden within the human soul requires us to creatively introduce the subtleties of wickedness without sustaining clichés. No easy task.
Making the beast exceptional requires the writer to look deep and forge an evil from the demonic influence on the human soul. Few writers can do this well. Perhaps even fewer want to do it.
God, for those of yours who are compelled to display evil, I pray your divine protection over them. I pray your insights will permeate their creations and that redemptive alternatives will be presented. In the Name of Jesus, Amen.