Rudolf Giuliani

There's an even stronger impulse driving some to the vigilante act than grandiosity: the realization that if you do nothing, you've essentially allowed yourself to be held hostage, not necessarily to the perpetrator, but to fear.

If spontaneous and proportional to the crime, the vigilante act, as opposed to the practice, is not a crime. In fact, there's no shortage of situations which call for taking the law into one's hands.