Before any given shooting, there may be no obvious distinction between who will become a hero and who will become a villain. Those who commence the killing—at the time and place of their choosing—may only seem like plausible culprits after the fact. What’s more, every vigilante is an action hero in their own mind. And most would-be avengers are inclined to see anyone who resists them as a villain.
The practical meaning of self-defense would not, in the absence of a common judge, be universally self-evident to everyone in every situation. This problem is what Thomas Hobbes meant by a “state of nature,” where “there will be a large number of cases where everyone must be their own judge of how and when to defend themselves.” In such a state of affairs, every person must be the private judge of “the necessity of the means, and the greatness of the danger” to their own lives. As the scholar Richard Tuck explained in his very short introduction to the Beast of Malmesbury:
By the terms of Hobbes’s account of the state of nature, conflict arises because people judge differently about what is a danger to them, and the fact that they judge differently is enough to show that there is an inherent dubiousness about the cases in question.
In these uncertain cases, there is no plain fact of the matter. Hence the need for a sovereign—a common judge that has the last word on what violence is legitimate.
The more judgements about “self-defense” are decentralized and the greater their scope, the more internecine violence we should expect. This problem is not reducible to mental illness alone. There needn’t be an epidemic of untreated mental disorders for you to find yourself surrounded by a legion of George Zimmermans who suspect they, too, are living in someone else’s gun sight.
update (December 24, 2012): David Frum wrote an excellent variation on this theme. Frum concluded that NRA President Wayne LaPierre’s vision for America is “one unending replay of the worst scenes in Charles Bronson’s 1974 vigilante classic, Death Wish.”
update (January 21, 2013): The suicidal danger of mistaking anarchy for liberty can be seen in the rolling disaster of Stand Your Ground laws.