On the topic of the current (or vaguely current anyways) conversation, there was a guy who went through and made a list of which demons he'd spare and which he'd kill, and why.
Nevart his name was. His post about the chapter 2 girls got nuked for some reason (or at least I can't find it), but I found myself pretty much agreeing with what he said.
In the end, I'm pretty sure there were only TWO spared out of the entire lot, namely Mukuro and Sphinx for being both (A) singular entities and (B) solely focused on Kazuya. The rest, whether for being too dangerous/risky to keep around or having killed far too many people, couldn't be spared.
Exterminating Barghest and Camazotz is basically just pest control (if pests were far more dangerous and could fuck humans to death)
Exterminating Lampas is a mercy killing more than anything (considering "lighting their lamps" is their entire life purpose, and SURELY they'd all be fine giving that up to co-exist with humans)
Exterminating Empuse, Haru & Liz, Nina, Arachne, Titanoboa, and Daji is just desserts considering how many they've killed (to varying degrees of course)
Exterminating Echo is risk management, plain and simple. The risk of getting a bad one far outweighs the chance of getting a good one.
Exterminating Mother Rabbit is definitely debatable, but even just the
chance that another person more could've been saved had she not been there...
Mukuro and Sphinx are the only ones who don't have outright confirmed victims, are singular entities instead of entire species, and they only go after Kazuya instead of trying to harm anyone else.
...Oh, right, just realized I forgot about Lucy. Uh... She was a 50/50 I think, depending on whether you think the people she killed in her previous life counts against her or not. I think he leaned towards spare on her though. So... three spares instead of two.
Still a fucking abysmal ratio of "kill to spare" though, so that should say something.