A murder in the Mexican state of Chihuahua last week horrified even hardened crime stoppers. Police Commander Martin Castro's head was severed and left in an ice cooler in front of the police station in the town of Praxedis with a calling card from the Sinoloa drug cartel.

According to Mexico's attorney general, 6,616 people died in drug-trafficking violence in Mexico last year. A high percentage of those killed were themselves criminals, but many law enforcement agents battling organized crime were also murdered. The carnage...