Didn't ISIS do that in Iraq and Syria? Kidnapping is also a common way for armed people to get money from others in insecure countries, possibly more than they could get by directly stealing their possessions.

ISIS didn't arise spontaneously after an earthquake. It was intentionally built over a period of 10 years by a coalition of political parties and religious movements as a continuation of the insurgent war against occupiers, with the eventual goal of establishing a new society with new laws: their mission was inherently social and cooperative (by means of military conquest).

That's hardly the prepper expectation of "there will be a solar flare and then the whole world will instantly be filled by roving gangs of psychopaths, like in The Walking Dead"