De Decalogo

Since, then, illicit cohabitation produces such great calamities, adultery is very naturally a detestable thing hated by God, and has been set down as the first of all transgressions.

The second commandment of this second table is to do no murder. For nature, having produced man as a gregarious and sociable creature, and the most easily domesticated of all animals, has invited it to a fellowship of opinion and partnership, giving him reason, as a means to lead to a harmony and admixture of dispositions. And he who slays any man must not be ignorant that he is overturning the laws and ordinances of nature, which have been beautifully established for the common advantage of all men.