In Book IV of Boethius’ The Consolation of Philosophy, Lady Philosophy memorably describes the bestial nature of evil:
It is goodness that raises a man above the level of humankind, and it therefore follows that evil thrusts a man down below the human condition, so that he no longer deserves the name man. One given over to vice is therefore no longer to be deemed a man. One who plunders others’ wealth is burning with avarice, and you could say that he is now a wolf. The wild one who is given to quarrels and lawsuits you could call a dog. The trickster with his cons and scams is a fox. The angry one who roars and cannot govern his anger is a lion. The coward who is afraid of everything is a deer. The stupid oaf is a jackass. The fickle one who follows his whims is a flighty bird. A man wallowing in his lusts is a pig. All those who have put goodness aside have no right to be called men anymore, since there is nothing divine about them, but they have descended to the level of beasts.