Hermit Kermit will probably have more info on this but the particularly shocking and excessive brutality in Mexico over the past 5 or 6 years seems to have started with the Zetas. Their whole approach has been to strike fear in the rivals, as well as the Mexican public in general. It started with the beheadings and then just escalated from there, almost like the warring cartels were trying to top each other. Before long, it wasn't just beheadings but people being hung from overpasses, dumped on busy streets, burned in barrels, etc. None of this is exactly new, as it's been seen with other organized crime groups elsewhere, but the sheer scale of it is unprecedented.