I wrote extensively about this in my book.
They are gruesomely and accurately captured in this book review from The New Yorker.
Child soldiers often rely on drugs to inure themselves to horror. Ishmael Beah became addicted to “brown-brown,” a mixture of gunpowder and cocaine. Cardona favored a cocktail of heavy tranquillizers and Red Bull, administered at regular intervals throughout the day, which rendered him alert but insensate. Miguel Treviño, though, required no drugs to kill. If the role he plays in “Wolf Boys” is an archetypal one—the psychopath father proxy, the charismatic comandante—the details have a chilling specificity. When Treviño is driving and sees a dog sleeping by the side of the road, he swerves to hit it. After stealing a tiger from the circus, he starves it, then feeds it human victims. At one point, Treviño tells Cardona that he has killed “more than eight hundred people.” Among the Zetas, this counts as a boast. It is not merely the act of killing but a real or feigned emotional indifference to the taking of human life that consolidates status in the cartel. Armed groups that use child soldiers often truck in mystical elements—one reason that Joseph Kony found kids so easy to manipulate is their readiness to believe in magic—and the Zetas betray some elements of a death cult. Cardona was not an especially spiritual kid, but like his colleagues he offered lip service to Santa Muerte, the Mexican folk saint of the dead.
In the Zetas, Dan Slater tells us, the highest praise you could offer someone was to say that he was frío—coldhearted. The first time Rosalio Reta kills someone, his comrades rally around to celebrate. “Your first job!” they exclaim. “You’re going to have nightmares!” He was sixteen. Slater charts Cardona’s evolution into an efficient and reliable killer, “a heat-seeking missile of black-market capitalism to be deployed against anyone who ran afoul of the Company.” At one point, Treviño touches Cardona’s chest and tells him, “You’re just as cold as me.”