I have watched enough cop and detective shows to know the standard line against vigilantism: some version of "You can't take the law into your own hands." But John Locke begs to differ. In section 19 of his 2d Treatise on Government: “On Civil Government,” he says quite explicitly that in situations where the police and courts won't do their jobs or are unable to do their jobs in time, that it is appropriate to take the law into one's own hands:
And here we have the plain difference between the state of nature and the state of war, which however some men have confounded, are as far distant, as a state of peace, good will, mutual assistance and preservation, and a state of enmity, malice, violence, and mutual destruction, are one from another. Men living together according to reason, without a common superior on earth, without authority to judge between them, is properly the state of nature. But force, or a declared design of force, upon the person of another, where there is no common superior on earth to appeal to for relief, is the state of war: and it is the want of such an appeal gives a man the right of war even against an aggressor, though he be in society and a fellow subject. Thus a thief, whom I cannot harm, but by appeal to the law for having stolen all that I am worth, I may kill, when he sets on me to rob me but of my horse or coat; because the law, which was made for my preservation, where it cannot interpose to secure my life from present force, which, if lost, is capable of no reparation, permits me my own defence, and the right of war, a liberty to kill the aggressor, because the aggressor allows not time to appeal to our common judge, nor the decision of the law, for remedy in a case where the mischief may be irreparable. Want of a common judge with authority, puts all men in a state of nature: force without right, upon a man’s person, makes a state of war, both where there is, and is not, a common judge.
There are three reasons to let the state take care of justice rather than taking the law into one's own hands that do not involve any mystical respect for the state. The first is the principle that "People Must Not Be Judges in Their Own Cases." And by the same token, my family, friends, close allies or someone in my pay should not judge my case either. There is too much chance of bias. But other than one's family, friends and close allies, whom can I convince to take an interest in justice for me? One can hope the state will.
Second, even if an impartial vigilante is available to take an interest in justice for me, there is a good chance that the vigilante's procedures for determining the facts and appropriate disposition of my case will be less accurate, and therefore less just than the state.
Third, even when a wise, impartial vigilante with excellent investigative skills is available to take an interest in justice for me, the state may object, simply as a power play. If the state isn't too much worse than the vigilante at securing justice, it might make sense to allow a modest degradation of justice in order to avoid getting in practical trouble with a state jealous of its own prerogatives.
All of these reasons to let the state take care of justice are a matter of degree. I consider the first the most compelling; people often do misjudge their own cases and the cases of those close to them, and even the cases of those in their own ethnic group, when the opposing side in the case is someone from another ethnic group. But once an impartial vigilante is in play, it is easy to get sympathy for that vigilante and the person who needs justice simply by portraying a situation in which the state is doing a bad job.
Moreover, when they are otherwise in the right, vigilantes who take the law into their own hands often get away with only light punishments. To me, this reflects the respect prosecutors and juries have for the principles John Locke writes of, whether those prosecutors and juries realize it or not.
Update: After reading this post, Dwight Allman makes a very interesting comment on Facebook about John Locke's shifting views of the state of nature. Here is a link.