Sometimes in murder mysteries, the plot will involve the murderer making a bad choice. Sometimes this is picking a bad time to put their plan into action. Sometimes this is thinking that something would work that wouldn’t, or predicting that someone would react to their plan in a way that they didn’t. Sometimes this is just coming up with a bad plan. So, what are we to make of this? Are any of them legitimate or are they all bad storytelling?

With the exception of a completely bad plan, I think that they can be legitimate, but I do want to elaborate on the counter-argument first. The most fundamental problem with the murderer making a bad choice is that it spoils the denouement. In the denouement, the detective takes the tangled mess woven by the intelligence of the murderer and sets it out, rationally, so that things now make sense. This is spoiled by the murderer making a bad choice because it is intrinsically impossible to give a good explanation for a bad decision. It is possible, of course, to give a good explanation for a good decision made upon mistaken premises which works out badly because the mismatch with reality has consequences, but that’s not a bad decision.

Against this, the writer of detective fiction must balance that every clue for the detective to find is, by definition, a mistake on the part of the murderer who does not want to be caught. The weakness of all detectives is the perfect murder—a murder in which no clues are left. The heart of detective fiction is that the perfect murder is not really possible since murder is wrong. Someone who uses murder as a tool is a fallen creature and fallen creatures do not commit only a single sin, since sin warps and deforms the soul. Very commonly this takes the form of the murderer assuming that he can control all circumstances so as to leave no clues and the world being out of his control intrudes and causes clues to be left, in effect punishing him for his hubris.

And here we see, I think, why it can be legitimate for the murderer to have simply made a bad decision: the murderer already made a bad decision in making the decision to murder someone, even apart from the morality of it, because the murderer should have known his limitations and that it is not possible to fully control the circumstances as he needs to in order to get away with it.

But not all bad decisions are created equally. A bad decision may be legitimate as far as the structure of the art form goes, but yet not be artistically interesting. The problem with a fundamentally bad plan is that it is irrational at approximately every level, and so there is nothing for the detective to explain. “And then he put hot sauce in the coffee because he thought it was poisonous. When that didn’t work, he bought his father another hat in the hope that two hats would cause his father to die of a broken heart. When that didn’t work, he tried dying the new hat green to give his father a heart attack by freight, even though his father was blind…” You could, perhaps, make this work in a slapstick comedy like the movie Murder By Death, but then Murder By Death was only sometimes funny.

I suspect that the line which demarcates artistically acceptable bad choices from artistically unacceptable bad choices is how commonly that kind of bad choice is made. Picking a sub-optimal time, under stress, to put a reasonable plan into motion is the kind of bad choice that anyone might make. Trying to use hot sauce to poison someone isn’t. The examination of partial mental breakdown is far more artistically interesting, because we all live among that, than is the examination of near-complete mental breakdown.