Given her common surname, Katharine Ann Olson’s murder will probably be best known as the Craigslist killing.

That’s the twist that gave the tragic story legs outside the Twin Cities — a young nanny pursuing a job lead from the online marketplace was lured by a 19-year-old who killed her.

But it’s worthwhile to remember that Craigslist was just a place put to evil use. The new angles fascinate gawkers, but the core of the matter is straightforward and very old.

Still, this story is a new thing, in its macabre way. Police say a young man living with his parents in the Minneapolis suburb of Savage — which it usually isn’t — lured Olson with an ad on craigslist.org. Authorities say Michael John Anderson posed as a woman, keeping up the pretense through a phone call with Olson.

When she showed up, he shot her in the back, put her in the trunk of her car, and parked it at a nearby nature preserve, police say. He got caught when they turned up Olson’s smashed cell phone in a park trash can, wrapped in a bloody towel marked with Anderson’s name. Olson wasn’t robbed, authorities say. She wasn’t raped. So far, no one knows why she was killed.

Craigslist users are wary, say reports. Of course: This dents the snuggly dot-org bonhomie of the site. If new ways are exciting, this shows they’re also dangerous if you don’t know where the creeps can hide.

Then again, such crimes can be done by old-fashioned want ads. A guy in Nevada has been booked for molesting boys he lured via craigslist. I wonder if people felt this sense of emergent danger the first time a molester prowled for victims via automobile. The mechanisms keep changing; the crimes don’t.

Which shows that it’s not really the mechanisms that count, even though they are where reformers turn first so often. No one’s proposed banning craigslist yet. Nor should they, but I wouldn’t be surprised if someone tried.

Consider Tyler Peterson, the 20-year-old in Crandon, Wis., who shot six other young people in October. Usually, after such mass shootings, you hear calls to ban whatever weapon was involved, but Peterson was a sheriff’s deputy, and his rifle apparently was official equipment. So talk turned instead to Peterson’s age and how well Wisconsin requires sheriff’s departments to do psychological screening.

We don’t know whether or not that would make a difference. Peterson was said to be steady-minded until his ex-girlfriend’s companions ridiculed him. Later, said a friend, Peterson was rational when he described what he did. Then he killed himself. Except for the two bursts of violence, this doesn’t sound like a lunatic.

He’s just a guy who snapped, as the saying goes. The urge to divine precisely what snapped him gets at the ultimate mechanism, the human brain. The phrase itself helps us think of mayhem mechanistically: If only psych screening could have detected the malfunction, then we’d have averted the horror. There’s the temptation to think murder must stem from sickness, from neurochemistry gone amok.

Oddly, you don’t get that mechanistic take for some tragedies, especially ones involving markets. In the subprime mortgage meltdown, there’s a swift popular assumption that someone must be to blame. Yet given how difficult it is for laymen even to understand what went wrong, it seems just as likely that insufficient caution and the erroneous pricing of risk, rather than someone’s sin, really do explain the distress.

Whereas people are quick to look for a faulty mechanism when some guy shoots someone else.

I speak hypothetically — Anderson only has been charged, not convicted. But given the lie he first told police, that he was present when a “friend” killed Olson just for fun, one suspects there’s something to the charges and that he might well have no better reason himself.

Anderson was evidently functional enough to hold a job fueling jets. This doesn’t suggest he was seized with mental malfunction. He may simply have chosen to do wrong. You don’t want to say he acted “inappropriately” — that sounds like noodle-spined parenting psychobabble — but that’s what it amounts to. He behaved inappropriately, to a needle-pegging degree. He did what he shouldn’t.

He did it while equipped, as a human, with free will and some sense that there are things you should and should not do. Fuel hoses go in the fuel port, not the ground. Bullets don’t go into human beings. If Anderson was capable of making the first distinction, we should presume he could make the second. To say, instead, that some psychological malfunction led to the trouble he’s in — that he’s sick, not bad, because of the inexplicability of his badness — is to treat him as a machine.

That doesn’t help anyone, least of all society as it measures out justice. The mechanisms of the case are interesting and new, certainly. But in the end, presuming the charges are true, what went wrong in the craigslist killing was something as old as Genesis: Someone chose to kill another. All that’s new are the means.

Making new rules and new cautions related to those means could cut the risk of a next time, but we shouldn’t imagine that down such paths lie ultimate freedom from the curse of humans who choose evil.

Mr. McIlheran is a columnist for the Milwaukee Journal Sentinel.