"We're talking about a demon here! A demon! A being that exists in darkness, a ruler of fear and destruction! The natural enemy of everything that lives! There's all pain here and no gain! I've been traveling with something lower than vermin!"
—Amelia, on learning the true nature of the 'mysterious priest' who joined(?) the heroes' adventuring group a couple of books back
"...Oh, stop... if you go that far even I'll be embarrassed...."
—Xellos, reacting to Amelia's inadvertent compliments
(Slayers 7: Gaav's Challenge)
It can be very difficult to insult some people.
So how does an insult work, exactly? The concept is simple enough: you call someone something they don't want to be called. The trouble is, how do you know what they don't want to be called? How do you, for instance, avoid complimenting someone totally, basically evil while still expressing your utter revulsion for them?
You could try calling them something good—an angel, a being of light, someone who's excellent at building people up and soothing their fears, something like that.
That would probably insult them, but it doesn't really get your disgust across, does it? There are two important things about an insult, after all: it has to be something the other person doesn't want to be called, and it has to be something that feels, to you, like an adequate expression of your negative impression. Calling someone like Xellos "lower than vermin" doesn't entirely relieve your feelings if you can tell, by their reaction, that they're flattered. And while calling Xellos an incarnation of goodness would probably have resulted in an insulted reaction, odds are good Amelia would've had mixed feelings about it.
So what do you do?
Well, in Xellos' case we know the solution: "生ゴミ." But why does this work? The answer, my dear readers, is in the 生.
生ゴミ (or "namagomi") literally means "live garbage." It's kitchen waste, food scraps, anything that rots especially vigorously. In namagomi, live microorganisms flourish in continual energetic acts of destruction, breaking what was once tolerable to the senses down into a putrescent, seething mass of utterly revolting life.
Q. But why should this be insulting to a demon/mazoku?
A. Because live garbage is so very, very alive.
Life is completely unacceptable to mazoku, so much so that just killing themselves isn't enough—they have to know that there isn't a single scrap of life left in existence... and yet they are themselves a form of life, flourishing in continual energetic acts of destruction. "Namagomi" works so well as an analogy for mazoku!
Is there any insult worse than one that's both repulsive and accurate?