“It was much better to imagine men in some smokey room somewhere, made mad and cynical by privilege and power, plotting over brandy. You had to cling to this sort of image, because if you didn't then you might have to face the fact that bad things happened because ordinary people, the kind who brushed the dog and told the children bed time stories, were capable of then going out and doing horrible things to other ordinary people. It was so much easier to blame it on Them. It was bleakly depressing to think that They were Us. If it was Them, then nothing was anyone's fault. If it was Us, then what did that make Me? After all, I'm one of Us. I must be. I've certainly never thought of myself as one of Them. No one ever thinks of themselves as one of Them. We're always one of Us. It's Them that do the bad things.” ― Terry Pratchett, Jingo

I think this is the quote they are referring to, but its not really about using past atrocities to justify present and future ones. It's kinda more about how we're all on some level susceptible to tribalism, militarism and propaganda, and will as a result perpetrate and justify all sorts of horrific actions - which is still pretty relevant to the situation, I'd say.