From Citizendium, the Citizens' Compendium
Revenge : An action intended deliberately to cause harm to a person or group of persons in response to an insult, affront, injustice (whether real or perceived.) The agent trying to take revenge (sometimes called vengeance) rarely has recognized authority to punish but acts outside the law. Revenge is generally viewed as a misguided attempt to re-establish equity and fairness, but it often backfires, fails, or causes more destruction than intended, and perpetuates violence. The sense of the term is activity that is not sanctioned by the judicial system, and is outside the boundaries of proper ethical conduct. When a criminal or terrorist is tried in a court of law and found guilty after proper procedures are followed, then the state's punishment of the lawbreaker is not viewed as revenge or vengeance but rather legitimate enforcement.