"An avidity to punish is always dangerous to liberty. It leads men to stretch, to misinterpret, and to misapply even the best of laws. He that would make his own liberty secure, must guard even his enemy from oppression; for if he violates this duty, he establishes a precedent that will reach to himself."
-- Thomas Paine, Dissertation on First Principles of Government (July 7, 1795)
Thomas Paine on extra-judicial killing.