In a Colorado town in 1996, Verle Mangum bludgeoned Janet and Jennifer Davis to death.

Janet, a 42-year-old nurse’s aide, had come home from work and discovered Mangum, then 17, having sex with her 11-year-old daughter, Jennifer. When Janet reached for the phone to call the police, Mangum picked up a baseball bat and fatally bashed her with it. He then used the bat to murder Jennifer as she cowered in the bedroom.

A jury eventually convicted Mangum of both murders. Under Colorado law he was given a mandatory sentence of life in prison without parole. In 2007, when the convictions were upheld on appeal, the prosecutor expressed relief. “This was one of the most heinous crimes in the history of our community,” District Attorney Pete Hautzinger told the Grand Junction (Col.) Free Press. “It is very gratifying to know for sure that he will be staying in prison for the rest of his life.”

But that’s no longer a sure thing – not after the Supreme Court’s 5-4 ruling in Miller v. Alabama last month that the Eighth Amendment’s ban on “cruel and unusual punishments” does not allow states to mandate life without parole for murderers who were minors when they committed their crime. Mangum, like at least 2,000 other juvenile killers serving mandatory life terms, including more than 60 in Massachusetts, will now have to be resentenced. The families of murder victims nationwide have now lost the reassurance that their loved ones’ killers would never be turned loose. Survivors may be forced to testify all over again. There is no telling how many monsters like Mangum will end up serving less – perhaps much less – than the life sentence the judge and jury imposed.

Hautzinger, outraged by the decision, said its impact would be “inhuman.” And when Mitt Romney held a town-hall meeting in Grand Junction last week, the DA showed up and asked him to comment on what the Supreme Court had done. But Romney sidestepped. He said only that he would “look at the particular case,” and that he favors “swift and severe punishment” for serious crimes.

What Romney should have said was that the court’s ruling was illogical and indefensible – a textbook case of justices turning their personal preferences into constitutional commands.

There is nothing uncommon about laws requiring life without parole for juvenile murderers, so such punishment can hardly be barred by the Eighth Amendment. As Justice Elena Kagan’s majority opinion concedes, legislators in 28 states plus the federal government have enacted laws mandating that penalty. Courts have meted it out thousands of times. Liberals may disapprove of mandatory “true life,” but it plainly isn’t unusual. By definition, therefore, it isn’t “cruel and unusual.”

Miller marks the third time in recent years that the Supreme Court has invoked “evolving standards of decency” to rationalize a new constitutional barrier to punishing minors convicted of terrible crimes. In Roper v. Simmons (2005), the court outlawed the death penalty for any criminal who was under 18 at the time of his offense. At the time, the majority observed that “life imprisonment without the possibility of parole” would remain an option. Yet now, in what the dissent calls “classic bait and switch,” the court tells states they may not insist on that option.

In Graham v. Florida (2010), meanwhile, the court ruled that juvenile offenders could not be sentenced to a lifetime behind bars for any crime but homicide, no matter how violent. (After all, said the court, “there is a line between homicide and other serious violent offenses.”) But now five justices declare that mandatory life sentences are unconstitutional for youthful killers too. And it’s only a matter of time, they hint broadly, until even discretionary life-without-parole is struck down.

In all these cases, the court has leaned heavily on social science and psychiatry. Since adolescents tend by nature to be more reckless and impulsive than adults, the majority argues, they cannot be assigned quite the same “moral culpability” for their deeds.

As a general rule, that’s obvious; it’s why the law distinguishes between adults and minors in the first place. But why should policymakers be prohibited from recognizing exceptions to the rule, and treating juveniles as adults in certain extreme circumstances?

Suppose, to go back to that terrible day in 1996, a 17-year-old had burst in on Mangum and prevented him from murdering Janet and Jennifer Davis. If public officials then wished to reward that teen for his life-saving courage, no one would suggest that the honor be diminished on the grounds that the hero’s adolescent brain didn’t fully grasp the significance of his valiant behavior. Most Americans would regard it as similarly unreasonable to suggest that any juvenile who acts with criminal depravity must be too mentally immature to understand what he is doing.

Supreme Court justices may personally disagree, of course. But to turn their disagreement into a constitutional imperative? To wrap their subjective views in the authority of the Eighth Amendment? That is inexcusable, and Mitt Romney ought to say so.

(Jeff Jacoby is a columnist for The Boston Globe. His website is www.JeffJacoby.com).