“There’s no greys, only white that’s got grubby. I’m surprised you don’t know that. And sin, young man, is when you treat people as things. Including yourself. That’s what sin is.’
‘It’s a lot more complicated than that -’
‘No. It ain’t. When people say things are a lot more complicated than that, they means they’re getting worried that they won’t like the truth. People as things, that’s where it starts.” ― Terry Pratchett, Carpe Jugulum
Another saying is something like evil is when you stop seeing other people as people and instead see them as things.
Yes it’s a discworld quote
That’s exactly what I was thinking of! Of course it’s Sir Terry.
Individuals must always be treated as ends, never as means. Animals are individuals, too.