Why did he do that?

Back in high school, I read a lot of mystery novels, many of which were police procedurals, and I got the basic triumvirate for figuring out who was the killer pounded into my brain: Means, motive, and opportunity. They actually apply to any villain undertaking any dastardly deed: the villain always needs a way to do it, a chance to do it, and a reason to do it. And of the three, motive is the most commonly neglected, to the extreme detriment of many otherwise-excellent stories.

I think there are a couple of reasons why motivation seems to get short shrift so often. For one, people do all sorts of peculiar things in real life for reasons that seem thin to the rest of us. It's always tempting for a writer to say "But I read in the news where this woman picked up a knife in a restaurant and stabbed the waitress because her tea was cold! So it's believable! It really happened!" But "it happened in real life" does not absolve the writer from the need to convince the reader that "it" happened in the story…and the whole reason incidents like that make the news in the first place is that they are strange and nearly unbelievable.

Another reason is because authors make the mistake of thinking that making a villainous character unpleasant and unlikeable is enough. While it is true that most people are more easily convinced that someone they don't like is behaving badly than that someone they do like is behaving badly, just being a nasty SOB doesn't automatically make someone willing to lie, cheat, murder, blackmail, or commit whatever other crimes one's villain needs to commit. Unlikeability is not a motive.

The third reason why motive gets neglected a lot, I think, is that villains often don't spend a lot of time on stage. They are, after all, trying to get away with their crimes without being caught or punished, and that often means keeping a low profile - and always means keeping the main characters from finding out that the villain had the means, motive, and opportunity to commit the crime. This means the writer doesn't have a lot of chance to show what the villain is really like, even if she knows - and a lot of writers learn about their characters by writing them, which means that a mostly-off-stage villain doesn't get to be known very well by writer or reader.

And a fourth is that the author sometimes realizes in mid-book that they can twist the plot in a new direction, if only some particular thing happens…and Character A is the only one who is in a position to take that action. So they have A do it, but they forget to go back and figure out why on earth he/she would want to.

A novel I read recently had what looked to me like several of these problems at once. The author had set up an intriguing mystery, plots within plots, and then about halfway through recomplicated things nicely with the discovery that the supposed villain was actually under the influence of an advanced drug that induced and mimicked the obsessive paranoia that can come with late-stage Alzheimer's. I was racing toward the finish when the discovery of the real villain, the man who had been secretly administering the drug, stopped me cold.

The real villain was, from a plot standpoint, an excellent choice. He had plenty of opportunity, and was one of the few who had the means (access to the drug). He wasn't onstage much, and he was quite unlikeable - abrupt, suspicious, impolite, and continually cranky. But I couldn't believe that he would feed a dangerous drug to a man he had supposedly been friends with for over twenty years, merely because he disliked the original and wanted to see them put in prison without getting his own hands dirty. It pretty much wrecked the book for me.

The author would have been much better off if she'd stopped to think more about why her real villain was doing what he did. She had two possible ways to make the book work (maybe more, but two that are blindingly obvious to me at the moment): she could have come up with a stronger reason than unfounded dislike and suspicion for her villain to go after his ultimate victim (a reason strong enough that the villain would betray an old friend in such a horrible way), or she could have turned the villain into a full-blown sociopathic personality who'd finally gotten to the point where he couldn't hide it.

The second choice is both the easier one to pull off and the one that would result in a weaker book. It would be easier to pull off, because it's perfectly reasonable that a high-functioning sociopath would have worked hard to conceal that condition, so when it comes out at the end that this is what is behind his actions, it's plausible for it to be a big surprise. It makes the book weaker, because it's too similar to what's already been done - the first "villain" turns out not to be responsible for his actions because of the drug, and the one who's really responsible is doing it because he has a disease. "Variation is good" applies to plot twists as much as to anything else in a story.

That leaves coming up with a stronger reason for the ultimate villain to drug his friend in order to get the friend to do what he wants done. This is difficult, but doable. Convincing the villain that his friend has betrayed him and deserves to be drugged, for instance, or giving him some dark event in his past that he can only cover up this way, or tying the friend and the victim together in some way that makes the villain see this as an appropriate revenge on both of them.

And it doesn't really matter whether one is talking about a murder mystery or a Regency Romance or an epic quest novel - if there's a villain, he'll be a lot stronger and more interesting if he has really good reasons for doing what he does.