There’s an interesting episode of the TV show Death in Paradise where one of the characters tells detective Poole to remember the 5 “BRMs,” the “Basic Rules of Murder”:

1. If it’s not about sex, it’s about money.
2. If it’s not about money, it’s about sex.
3. A wife is always most likely to kill a husband.
4. A husband is always most likely to kill a wife.
5. The last person you should discount should be the one you least suspect.

This is, obviously, an incomplete list; among other things it says nothing about revenge. It is surprisingly complete, though, for being such an incomplete list; especially the first two cover the vast majority of murders in mystery fiction. If one were to inquire into this it would be a chicken-and-egg problem, since seeming rational and being guessable are two criteria for the murders in murder mysteries.

It would be quite possible to have murders where someone picks names out of a phone book using dice, but these would be effectively unsolvable, and moreover, uninteresting. They are the domain of horror stories, not mystery stories.

This requirement for being rational and guessable does limit the scope for murder considerably, and hence why the first four BRMs are so widely applicable. So when considering other motives for murder besides sex and money, the murder mystery writer needs to consider whether they can be made to fit these criteria.

Revenge is obviously a possible motive that is both rational and guessable, but I’m wondering if it is possible to make a murder work that is, essentially, useless. Not purely random, of course, since that would satisfy neither criteria. But a murder where no one benefits.

The three ways that this has been worked, that I’ve seen, are:

1. Where someone does benefit, but the benefit is secret.
2. Where someone thought that they could benefit, but turned out to be wrong.
3. Where someone benefits, but the benefit is not widely regarded as a benefit.
4. Nobody actually dies.

An example of the first would be a Brother Cadfael story in which the murderer was the bastard son of the victim, but the manor was in Wales where bastards can inherit (provided the father acknowledges paternity). The location of the manner together with this quirk of Welsh law were not known to any single person (and hence to the reader) until the end of the book.

I’m having trouble thinking of a specific example for the second case, but I’ve seen several cases where the murderer expected to inherit from the death but turned out to not be in the will.

An example of the third would be the death of the American millionaire in The Secret Garden. Valentin killed him to keep him from putting large amounts of money into the promotion of the Church in Europe; that an atheist could care that deeply about the cause of atheism was not widely credited by those who were not Father Brown.

An example of the fourth would be a person killing off merely an identity of his, in order to take up a new identity elsewhere. Admittedly, this is often about money in the sense of escaping debts, but it can be done for other reasons. In one Sherlock Holmes story it was actually done as an attempt at murder, by framing the intended victim for the fake crime. This is also a way of making in a random murder intelligible, because the one faking his own death frequently supplies an unrecognizable corpse to make the story convincing.

The first of these methods is probably best classified as being about murder or sex, so I’m not sure, in the end, I should have included it. It is, however, important to keep around as a way of disguising the others.

The case of a person thinking that they will benefit from a murder, there does of course need to be some sort of rational reason why a person might have had this expectation. A mistress who was fed lies by a married man, a cult who thought that someone was more in their power than was, or even a wife who didn’t know about a mistress could all do it. That last, though, does illustrate a problem with the approach—the benefit has to be someone no one else would expect, or it’s irrelevant that the person didn’t actually benefit. A wife who was cut off without realizing it would be a normal suspect.

Someone who expected to benefit in a will is probably the most common example, but I think that there can be others. I know that there was an Agatha Christie story in which someone didn’t benefit from a murder because the actual mechanism was uncertain and so didn’t actually kill the victim until after the victim had written the murderer out of her will, and informed her of it.

The same can also work for a sexual motivation, of course. A person who kills a rival only to discover that the object of their affection won’t choose them even when free of their spouse.

Still, it seems that there must be some way to have another motive than expected sex or money. Power and prestige can work, I think. Though really this just gets us back to the beginning, in finding alternatives. But it’s worth pursuing. Bishop Barron noted that Saint Thomas identified four things a fallen human being can substitute for the love of God in this life:

1. power
2. pleasure
3. wealth
4. honor

Sex can, roughly, be identified with pleasure, in this list—though in some ways it’s more complicated than that. Wealth and murder for money are obviously connected. Power and Honor seem far less common than the other two.

The relative paucity of killing for the sake of power may be related to the commonality of democracy in the modern world, together with the way that people switch jobs so commonly in the modern economy that it would be hard to envision someone killing for one.

I do not think that this is an insuperable barrier, though; there are plenty of jobs at which a person only really has one shot in their life. Academic jobs are a good example; they are incredibly hard to come by, these days. At the same time, they are also hard to guarantee getting; it is not easy to have a guaranteed line of succession. That can play into the “falsely expected to benefit” angle.

Control of a business can work for this purpose; it may be enough to dilute a foe’s control by having his shares spread among his descendants. Even killing a competitor can be sufficient for this purpose. As soon as I say that, these do pop up more often, at least recently, as red herrings—theories which a bull-headed police detective clings to while the detective pursues the real theory.

And, to be fair to this approach, we live in a time when people’s lives are guided to an extraordinary degree by their crotches. In some sense, making all murders at the direction of people’s genitalia has a certain essential realism about it.

I don’t think that this realism is worth it, though. Mystery fiction is intrinsically unrealistic, and one of the legitimate purposes of reading fiction is to escape, for a time, to a better world than this one, where we can refresh ourselves to rejoin the fight in this world. I think that can apply to murders, too—to live, for a time, where people murder for better reasons than The Crotch Shall Not Be Denied.

With regard to honor, I have definitely seen this in the form of people killing blackmailers and whistle blowers. Gaining honor through murder is much rarer, from what I’ve seen. It’s nowhere near as easy to accomplish, which makes it a curious subject to think on. It may have the problem that gaining honor necessarily involves fame, which means that it cannot be quiet—and I prefer quiet mysteries to ones with high stakes. Still, both Conan Doyle and Agatha Christie managed to pull it off that the detective was quietly in the shadows, so this is not a fatal objection.