Picking villains is tricky.
I am working away on book 2, A Dollar Short, and I have come to one conclusion: villains are hard.
I think this may be particularly true in mysteries. To play fair with your readers, you have to depict the villain early, and the villain has to have all the potential villainy available from the beginning. It doesn’t have to be obvious, but it cannot be impossible for them to grow into the role.
I am a little conflicted by some other issues around them as well. In the modern world, all your characters fall under the lens of fairness of portrayal. And here I do not mean just fairness to the reader, but rather are you fairly portraying society, races, genders, and so forth? That can be a tricky question. Certainly some authors get nicked for not portraying, say, strong females. Or strong people of color. Or strong whatever. Pick your favorite marginalized, picked-upon, or unfairly treated group. And others are lauded for doing so. Think about, say, Joss Whedon’s reputation around Buffy the Vampire Slayer.
You can see this in other genre work as well, Anyone familiar with recent events in, for example, science fiction, will be familiar with the war between the “sad-puppies” and the “social-justice-warriors”. Our current culture is full of disagreement about how we should portray ourselves, what we should talk about, who should be shown as strong, and so forth. That can make picking a villain tricky. People are bound to notice choices you make. Characterizations of whatever sort are very revealing about who and what the author is or believes (or can be). Does constantly picking the same sort (see the above list) for a villain indicate you feel a particular way toward that sub-group?
Yet we are always encouraged to write what we know. And how many of us have real understanding of cultures that are not our own? We can learn, examine, and question other cultures, but we cannot live them. A man cannot experience being a woman any more than a woman can experience being a man. I cannot be black and understand things from that point of view, no matter how much I read, learn, study, question, or research.
And sometimes I find some character just demands to be the villain of the piece. Every action, every thought, every portrayal ends in the same place, with the same motivations for some characters. Just as some heroes (or she-roes) cannot help being heroic, I think some villains are born to villainy. How can you force a character into a mold without the character seeming, well, molded?
I guess, in the end, you have to show fair motivations for both your heroes and villains. They should all have viewpoints, and all the viewpoints should be, if not correct, at least understandable. In the end, the human experience is human. You have to be able to see the character as an actual person, however flawed, broken, or tormented. You might not agree with their take on the world, but you have to be able to perceive how they might come to that view. Otherwise the character falls flat, and the work seems contrived. They become Snidely Whiplash or Dudley Doright rather than a fully formed person.