The short version:
Sin, young man, is when you treat people like things. Including yourself. That’s what sin is.
And the full version:
“…And that’s what your holy men discuss, is it?” [asked Granny Weatherwax.]“Not usually. There is a very interesting debate raging at the moment on the nature of sin. for example.” [answered Mightily Oats.]“And what do they think? Against it, are they?”“It’s not as simple as that. It’s not a black and white issue. There are so many shades of gray.”“Nope.”“Pardon?”“There’s no grays, only white that’s got grubby. I’m surprised you don’t know that. And sin, young man, is when you treat people like 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.”“Oh, I’m sure there are worse crimes–““But they starts with thinking about people as things…”–from Carpe Jugulum, by Terry Pratchett.
Why do I love this? I think because it explains very simply something that I have long felt to be true: that is, how people can treat each other so badly. It’s about lack of empathy, and an inability to understand that other people are human too – are human first, in fact, and different second. And it’s a message that I think resonates very strongly right now.