A man who, without exercising public authority, kills an evil-doer, shall be judged guilty of murder, and all the more, since he has dared to usurp a power which God has not given him.
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic evil murder
Date 1274
Language English
Reference
Note Translated by Fathers of the English Dominican Province
Weblink http://www.gutenberg.org/cache/epub/18755/pg18755-images.html

Context

“Now the slaying of evildoers is useful for the common good, as stated above (A. 2) . Therefore it is deserving of praise if even private individuals kill evil-doers. On the contrary, Augustine says (De Civ. Dei i) [*Can. Quicumque percutit, caus. xxiii, qu. 8] : "A man who, without exercising public authority, kills an evil-doer, shall be judged guilty of murder, and all the more, since he has dared to usurp a power which God has not given him." I answer that, As stated above (A. 2) , it is lawful to kill an evildoer in so far as it is directed to the welfare of the whole community, so that it belongs to him alone who has charge of the community's welfare.” source