Lactantius

For when God forbids us to kill, he not only prohibits us from open violence, which is not even allowed by the public laws, but he warns us against the commission of those beings which are esteemed lawful among men....Therefore, with regard to this precept of God, there ought to be no exception at all, but that it is always unlawful to put to death a man, whom God willed to be a sacred animal.

For they, the philosophers, were considered teachers of right living, which is far more excellent, since to speak well belongs only to a few, but to live well belongs to all.