The usual arguments for and against capital punishment will be trotted out this month as mass murderer Dzhokhar Tsarnaev, filled with hatred for Americans fueled by radical Islamism, faces sentencing for the 30 crimes for which he has been judged guilty.

On the one side are those who believe that just because someone brutally rapes and murders an innocent woman or murders and maims more than 260 people does not mean we in turn should respond as if we are ourselves equally uncivilized.  It is up to us to exhibit a higher level of humanity and ensure that we set an example of respecting human lives, even though these perpetrators did not.  In this way we separate ourselves from them and their depraved acts rather than lowering ourselves to their level of cruelty.  Indeed, for those who make this argument, no act of murder, genocide or vicious cruelty is so evil as to warrant our decision to take another human being’s life, or even to see those perpetrators of cruelty die anything but a quick humane death if that is what the law requires.

On the other side are those whose logic compels them to believe that only a hopelessly naïve or frighteningly misguided society would want to see such vicious killers free to live, to study, and to be provided food, shelter and health care at taxpayer expense for the rest of their lives.  Even the staunchest opponent of capital punishment, once victimized or having lost a loved one at the hands of truly evil miscreants, typically fall into this group. 

Those who hold this view believe, first and foremost, that the victims and their families deserve closure and closure does not come from incarceration that allows the assassins to marry, rally journalists to their cause, and tie up the court system with frivolous demands.  Those in favor of capital punishment believe there are some crimes so egregious that it is a society’s responsibility to remove a cancer from the body politic.  Once a criminal has been witnessed committing a horrendous crime, freely confessed and given a fair and open trial by a jury of his peers, it matters not that he suddenly “feels bad” about what he did.  He had free will, and he willingly chose to brutalize others.

To these arguments I would add another: the odds of such a cancer metastasizing.  Dzhokhar Tsarnaev is a cold-hearted killer who shows no remorse for his acts whatsoever.  He is so thoroughly indoctrinated into the principles of violent Islamism that “deprogramming” is beside the point.  The point is: what is he likely to do with his remaining half-century or so in prison?  With whom would he interact, and how?  Beyond the addled women who will want to mother him because he’s so cute, or those who want to bear his children because they share his hatred, his primary interaction as the years go by will be with men younger than himself, many of whom are already fascinated by violence and have a rage burning inside against they know not what.

Who better to channel that rage than a man like Tsarnaev?  He will be free to preach his vitriolic hatred for the next 50 years.  And he can preach it to men who already want to commit evil deeds and seek only a justification for so doing.  Their justification might easily be their conversion to a repellent belief that preaches that murder at its most extreme is commendable and condoned as long as it is “for the cause.”  In this case, converting felons who were “merely” remorseless dope dealers, ruthless pimps or sadistic bullies into remorseless dope dealers, ruthless pimps or sadistic bullies who believe they are committing their new atrocities for the betterment of their new religion. 

Rational people expect others to think as they do.  This is a fallacy. If we harbor any such illogic, we need only listen to self-appointed ISIS leader al-Baghdadi, when he says, “how can …Bush, Obama, and Kerry obstinately claim that ‘Islam is a religion of peace,’ meaning pacifism?”  He quotes the Qur’an in saying Islam is the religion of the sword, not the religion of peace, then says: “And fight them until there is no fitnah and the religion, all of it, is for Allah.” Pretty straightforward thinking: kill everyone who does not submit to their idea of God.

Or listen to Zubeidat Tsarnaeva, the coddling mother of the two men who killed and maimed that fateful day a year ago at the Boston Marathon.  She has steadfastly maintained the illusion of both sons’ innocence, just two weeks ago claiming on Russian social media, “America is the terrorist and everyone knows it! My son is the best of the best.”

A man whose brain is filled with the heartless blood lust of an al-Baghdadi and the cosseting countenance of a mother who encourages deviant behavior will not change his ways via incarceration.  That would be fine if it went only that far.  But if he is then allowed to provide equally seductive lies to tens or hundreds or thousands of acolytes within prison walls who, unlike him, will be paroled and set free to create similar or worse carnage, then society will have made a terrible mistake only some will live to regret.  The most important reason to continue capital punishment for the most heinous of crimes?  Dzhokhar Tsarnaev, and those who will surely follow.

Mr. Shaefer, a retired U.S. Air Force brigadier general, is a senior geopolitical consultant with Omnis Inc.