Valid Vigilantes

A vigilante (from the root "vigilant") is one who asserts his sovereign authority to enforce justice.

Nothing more, nothing less. Note that justice is a separate concept from legality. Legality is defined and enforced by a legal system. Justice is a moral concept and is defined and enforced by individuals. Although individuals enact the collective body of legal authority through law enforcement officials, the illusion of vigilantes acting "outside the law" is, in fact, a perceptional flaw.

Vigilantes act within the confines of a higher and stronger "law" than that of the legal system. They are compelled by their very nature to enforce justice, though the heavens may fall.