A criminal who, having renounced reason … hath, by the unjust violence and slaughter he hath committed upon one, declared war against all mankind, and therefore may be destroyed as a lion or tyger, one of those wild savage beasts with whom men can have no society nor security. And upon this is grounded the great law of Nature, "Whoso sheddeth man's blood, by man shall his blood be shed."
Second Treatise of Civil Government, Ch. II, sec. 11 - Two Treatises of Government (1689)