“If there is such a phenomenon as absolute evil, it consists in treating another human being as a thing.”

The Transhumanism Handbook, 2019

Variant translation: Now slavery has a certain likeness to death, hence it is also called civil death. For life is most evident in a thing's moving itself, while what can only be moved by another, seems to be as if dead. But it is manifest that a slave is not moved by himself, but only at his master's command.
Chapter 14 https://www.pathsoflove.com/aquinas/perfection-of-the-spiritual-life.html#chapter14
On The Perfection of the Spiritual Life https://www.pathsoflove.com/aquinas/perfection-of-the-spiritual-life.html (1269-1270)
Original: (la) Vita enim in hoc maxime manifestatur quod aliquid movet se ipsum; quod autem non potest moveri nisi ab alio, quasi mortuum esse videtur.

Variant: And sin, young man, is when you treat people like things.
Source: I Shall Wear Midnight

"Gun Violence? No! Goon Violence," http://www.unz.com/imercer/gun-violence-no-goon-violence/ The Unz Review, September 4, 2015.
2010s, 2015

2011, Tucson Memorial Address (January 2011)
Context: We may not be able to stop all evil in the world, but I know that how we treat one another is entirely up to us. I believe that for all our imperfections, we are full of decency and goodness, and that the forces that divide us are not as strong as those that unite us.
Context: I believe we can be better. Those who died here, those who saved lives here — they help me believe. We may not be able to stop all evil in the world, but I know that how we treat one another is entirely up to us. I believe that for all our imperfections, we are full of decency and goodness, and that the forces that divide us are not as strong as those that unite us.

Source: De Veritate (On Truth) q. 1, art. 2, ad 4

Bk. 2, Ch. 4
Confessions of Felix Krull, Confidence Man (1954)

Guardian interview (2006)