If we consider a man in himself, it is unlawful to kill any man, since in every man though he be sinful, we ought to love the nature which God has made, and which is destroyed by slaying him.
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic love God
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

“On the contrary, It is written (Ex. 23:7) : "The innocent and just person thou shalt not put to death." I answer that, An individual man may be considered in two ways: first, in himself; secondly, in relation to something else. If we consider a man in himself, it is unlawful to kill any man, since in every man though he be sinful, we ought to love the nature which God has made, and which is destroyed by slaying him. Nevertheless, as stated above (A. 2) the slaying of a sinner becomes lawful in relation to the common good, which is corrupted by sin. On the other hand the life of righteous men preserves and forwards the common good, since they are the chief part of the community.” source