On the use of beheadings by the Zetas drug cartel in Mexico, from El Narco by Ioan Grillo:

"It is still unclear exactly what inspired such brutality. Many point to the influence of the Guatemalan Kaibiles working in the Zetas. In the Guatemalan civil war, troops cut off heads of captured rebels in front of villagers to terrify them from joining a leftist insurgency. Turning into mercenaries in Mexico, the Kaibiles might have reprised their trusted tactic to terrify enemies of the cartel. Other point to the influence of Al Qaeda decapitation videos from the Middle East, which were shown in full on some Mexican TV channels. Some anthropologists even point to the pre-Colombian use of beheadings and the way Mayans used them to show complete domination of their enemies."