MEDELLÍN, Colombia — Most everyone agrees: The only thing worse than killing is being killed. If our lives are threatened, we have the right to defend ourselves, with force if necessary. In a civilized society, that defense is delegated to the state. But not all of us, apparently, live in that kind of civilized society.

Colombia in the 1990s saw the rise of vigilante self-defense groups. In its impotence and desperation at not being able to rapidly win the war against the guerrilla army (which was essentially a drug cartel) and against the drug lord Pablo Escobar’s private army, the state gave the green light to these groups — called Convivir. They were made up of agricultural laborers, trained by soldiers, and financed by landowners and agribusinesses. When they began to extort money from the very businessmen who were financing them, they were declared illegal. But it was already too late. They had become clandestine paramilitary groups, using the same weapons as those they were fighting: kidnapping, murder of innocents, drug trafficking.

What has been going on these last few months in Mexico, in the western state of Michoacán, makes me fear that the same thing is happening there today. “Autodefensas” have organized to drive out the vicious local drug cartel, called the Knights Templar. After first demanding that the vigilantes disband, the government of President Enrique Peña Nieto has now sanctioned them as part of the Rural Defense Corps — at least nominally under the control of the military.

This is how it happens. The army, with the blessing of the central authorities, looks for an ally, a lesser evil among the local powers. Compared with the cruel and bloodthirsty Knights Templar, the self-defense groups have popular support and are allowed to operate. Meanwhile, the government ignores the fact that some of these vigilantes might be financed by the enemies of the Knights Templar — for example, rival drug gangs or another cartel from the neighboring state of Jalisco. The government allows the vigilantes to act for a while, but when it tries to come back in, the self-defense groups will have turned into a real armed power with whom the government will have to make a pact, for without them the state won’t be able to assert its authority.