In February 2012, a young, beefy Egyptian named Islam Yaken took a shirtless selfie and posted it on a Facebook competitor called vk.com. The picture wouldn’t have attracted attention outside his circle of friends, were it not for the photos of himself he tweeted two years later. In that time, the Wahlberg wannabe with tidy, cropped hair had transmogrified into a bushy-haired hipster with heavy-rimmed glasses--who had gone to fight for ISIS. The jihadi accessories in his new photos included a Kalashnikov, a sword, and a bucket of Shia heads.

When Yaken’s pictures went viral a month ago, they provoked confusion about how this well-educated, urban gym-rat could so rapidly embrace a group known for its austere, backward-looking form of desert Islam. The same confusion reigns over the transformation of Abdel-Majed Abdel-Bary, the former stoner who rapped in London under the name “L Jinny” and is now a suspect in the murder of journalists James Foley and Steven Sotloff. There is, of course, an obvious continuity between the Yaken who yanked his towel well below the pube-line to give a full glimpse of his abs and the narcissistic poseur now in Syria, as well as a thread of miscreance that runs through the life of Abdel-Bary.

But not everyone who joins ISIS is the type to trade protein shakes and doobies for scimitars and explosive belts. Patrick Skinner, a former CIA case officer now with the Soufan Group, estimates a total roster of 10,000 to 15,000 ISIS fighters, of whom a small fraction may be foreigners. By now, a loose taxonomy of ISIS supporters has emerged, and in it, we see how the group’s rapid expansion also makes it vulnerable.

Before going to Syria, Yaken took a religious turn. But both he and Abdel-Bary appear to have more taste for grindhouse than Islamic jurisprudence—and that makes them exemplars of the most lurid and photogenic of the three types of ISIS fighter. Call them the Psychopaths. Skinner says the foreigners tend to be hyperviolent, and the indigenous fighters (and the local population who passively supports them) saner and more practical. One need merely look at Yaken’s sword-wielding photo to note its theatricality: The blade is a fantasy design, half hunting knife and half Chinese dao, with hooks, a teardrop-shaped hole, and serration that serve no function but to look cool. And that, of course, is the point. As men without significant military training—like most jihadis from Western or upper-class backgrounds—their main purpose is to create grotesque propaganda and, perhaps, to perform the low-skill role of blowing themselves up.

The second group is more pious. Call them the True Believers. They are drawn to the caliph himself, Abu Bakr al Baghdadi—a man with a deep, if also horrifying and heterodox, understanding of Islam. Among the Europeans who have flocked to Raqqa, Syria—the seat of the Islamic State—one sees not just blood-spattered young men but families. (In fact, Yaken, crossing taxonomic categories, encouraged his mother to immigrate.) Many of these people have come to Syria out of an inalterable sense of authentic religious obligation. According to The Independent, ISIS boasts a Tunisian with a doctorate in telecommunications to run its phone grid. It would seem safe to assume that a man immersed in this bureaucratic task does not also have a penchant for producing gore-porn.