The latest video from the Islamic extremist group ISIL takes inhumanity to another level by setting a captured Jordanian pilot (caged and doused with flammable liquids) on fire. Apparently mass shootings and beheadings have grown tiresome for ISIL.

And as always, the question is asked: who would ever do such a thing? How does someone go from being in school, playing sports, or working full-time, to joining a grotesque mass movement that kills with fetish-like frequency and reptilian indifference?

This is not just a rhetorical question. As we in the West are finding out, thousands of individuals from the United States, France, England, Germany, Sweden, and elsewhere have flocked to join ISIL, which seeks to establish a worldwide caliphate by using the techniques mastered by the Gestapo 70 years ago.

The answer lies in history and the findings of an intellectual giant working as a longshoreman. First, the history. As Alexander the Great learned early on, nothing gets like killing and slaughter. Do it with enough ferocity and whole cities and kingdoms will yield even before you arrive. In Alexander’s day, it was enough to allow a few survivors to escape and flee to the next city for the propaganda to work. Word of mouth was the Internet of the day. ISIL has perfected the more modern version (their videos are masterfully put together by professional videographers and editors—even with sound tracks). They use videos of their atrocities to terrify—which explains the millions of refugees who have fled Syria, Iraq, and elsewhere—but also to recruit. While most of us understand the former, the idea of recruitment via mass killings and beheadings is hard to fathom.

Here is where the absolute brilliance of Eric Hoffer, the moral and social philosopher and yes stevedore comes in. In 1951, Hoffer published The True Believer, a classic that should be widely read and studied. As an observer of mass movements, Hoffer did well to point out that mass movements attract people because they provide hope. In the case of ISIL, that hope takes the form of the professed intent of ameliorating Muslim grievances against the West and the prospect of establishing a worldwide caliphate to once more rule over Muslim lands and people. As flawed as those may seem to us in the West, most people can understand how they might prove influential.

Hoffer took his analysis a step further by recognizing that the marginalized, the desperate, the unemployed, the socially wounded, or the traumatized are naturally attracted to mass movements because the collective affiliation brings meaning into their lives, along with the prospect of hope. This, too, can be understood, even though it is a course most of us would not follow.

But perhaps most disquieting, Hoffer found another kind of individual who is attracted to mass movements—in fact, whose participation is often necessary for such movements to thrive. He referred to these individuals as “The Sinners.” We have to excuse Hoffer’s language here with the understanding that he was not schooled in psychology, but his terminology detracts in no way from his accuracy in deciphering mass movements. What Eric Hoffer found, and what has often been overlooked by many sociologists and certainly by the general public, is that mass movements attract what we now call the —in essence predators: individuals who are content in causing great harm, who perhaps are even sadistic, and yet aren’t bothered in the least by what they do.

Hoffer’s observation should not come as a great shock; after all, the Nazis had psychopaths in droves. Under the Swastika banner and while wearing brown shirts, they did horrible crimes without any regret—remember Kristallnacht?

Mass movements that utilize violence as part of their solution attract and even need psychopaths. After all, someone has to carry out the violent acts, the beheadings, the mass shootings, and the setting of humans on fire, even as these individuals plead for their lives. They need people who are callous, who can hurt others effortlessly because they have no . They need people who are immune to the pleas of a grieving mother and who view a crying child as an inconvenience that also must die.

Psychopaths are not like you and me. While you and I might look for love and success in life, the psychopath looks for exploitable weakness and opportunity. For psychopaths, a mass movement, especially one cloaked in religion, provides the opportunity to do anything they wish, uninhibited by morality, laws, or the police. For them, an especially violent mass movement, whatever the being espoused, is like being at a diabolical theme park where stealing, raping, killing, shooting, or decapitating humans is purposeful as well as recreational.

In violent mass movements, moreover, the more violent and virulent a member is, the more respect is garnered from fellow true believers and those likely to be recruited. This is exactly what transpired with Abu Musab al-Zarqawi. It wasn’t enough to join al-Qaeda and be involved in bombings. As grotesque and unconscionable as the videotaped beheadings of Nick Berg and Eugene Armstrong were, al-Zarqawi’s reputation and stature increased with each—and so did al-Qaeda’s recruitments. Unbridled psychopathy creates unbridled infamy. That is why we remember Stalin, Eichmann, Pol Pot, the Boston Strangler, and Ted Bundy.

All of this is disturbing on many levels—including the fact that little of it is new. We have known for a long time who joins mass movements and why—yet governments, politicians, and even agencies seem surprised when current events confirm it time after time.

The more recent manifestations are cause for additional concern because unlike 70 years ago when most of the Nazis came from one general population from a particular geographical area and their recruitment was based on word of mouth, mass rallies, or newspaper accounts, the internet now casts a global net. That net, filled with , hatred, violence, and well-produced theatrical brutality, is attracting thousands. They are not all psychopaths, to be sure, but enough are. Their cruelty will rub off on others, and unfortunately no government or mother pleading for her child to be released will prevail, because psychopaths have no conscience. They have no remorse, as Robert Hare, the world's leading expert on psychopathy has so often warned us. You can plead for a snake not to bite you, but reptiles do as they please—so do psychopaths.

Since World War II, we have had to live with the infamy of the Khmer Rouge, Serbia, Bosnia, Herzegovina, and Rwanda, among many others. All serve to remind us that mass movements attract the criminally facile, the remorseless, the unfeeling, the anti-social personality—what Eric Hoffer called “The Sinners,” we know them today for what they are: psychopaths.

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Joe Navarro, M.A. is 25 year veteran of the FBI and is the author of What Every Body is Saying, as well as Louder Than Words. For additional information and a free bibliography please contact him through Psychology Today or at www.jnforensics.com – Joe can be found on Twitter: @navarrotells or on Facebook. His latest book Dangerous Personalities (Rodale) is available on Amazon. My and thanks to Thryth Hillary Navarro and Toni Sciarra Poynter for their much appreciated assistance in editing this article which is based on the Predator chapter in Dangerous Personalities.