For me, the villains in my stories need to be at least as realized as the heroes. Now, that doesn’t always make it onto the page, but it is always there in my notes, in my head. I know why Robert does the things he does in A Long Walk (if you’ve read it you knew Robert was the villain right away right?), I know how the preacher came to build his cult in Resource Economies. In fact, I know more about them than I do about the heroes.
Now, my villains so far in this series have been insane, but it’s true insanity. They are acting with the rationale of their madness, everything they do is justified to them. Insanity isn’t random, often insanity will lead to more predictable behaviour. I have based these villains on actual people with violent forms of mental illness. Please note: my protagonists may also have mental illness. What makes my villains act the way they do is a combination of mental illness and circumstances. Robert outside of a zombie apocalypse is a bad person, a bit of an asshole, but that’s about it. He doesn’t become truly frightening until the rules of society go away and he is now free to act as his broken, twisted psyche desires, which feeds on itself.
The preacher is a bit different (I’m calling him that because I already know I will be changing his name in editing, but I haven’t decided to what – part way through the book I realized his name was too close to the protagonists). He was actually broken by the apocalypse, his religious mania is created by this world. That isn’t totally clear, although I do think I will add it a bit more.
The point is my villains have real reasons, real issues.