Mexico's first paramilitary "self defense" group: Mata Zeta (Zeta killers).
Politics abhors a vacuum. When the state loses the capacity to provide security for its citizens, they commonly seek to provide it for themselves. This basic fact of politics plays out time after time in weak and failing states. In Colombia, for example, during the 1980s and in response a growing, drug-fuelled insurgency, a group called "Muerte a Secuestradores," (Death to Kidnappers) was formed. It quickly blossomed into a full blown paramilitary movement that in most respects is a bigger threat to the Colombian state than even the guerrillas.
Mexico is not a failed state, or even a "weak" state by most standards. This Mata Zeta group may be an isolated phenomenon, or it may be symptomatic of the decline of the state's monopoly of violence. Some might applaud the initiative of the Zeta Killers. If the latter, we might expect the movement of Mexican "self defense." Of course, armed groups do not get far without funding, organization, and followers. The temptation of any armed group would be to prey upon the drug trade, as did the Colombian paramilitaries. In the end, they became just another self-interested movement playing the politics and economics of civil war.
Of course, it still remains to be seen whether Mata Zeta is a real paramilitary group. Some speculate it could be a covert government operation; others say it might be rival drug gang.