So I guess what I'm awkwardly saying:
I'm just not 100% sure that if Star somehow could communicate with the spells and told them "Guys I've done some soul-searching and I think it would be better for everyone if magic didn't exist, are you cool with that?" she'd get a response like Narwhal or whoever accepting the inevitable with an attitude of "ours is but to do and die."
Also I guess another reason I don't quite see it as genocide, or at worst deliberate genocide, is that (1) Star wasn't really aware of the lives the spells had inside the wand. She doesn't talk to them. They know her but she doesn't know them. (2) Star's brains were scrambled good in the Realm of Magic so she's probably not thinking too hard about the fate of the unicorns either. (3) Like I said, the MHC were meant to be "villains," and Glossaryck explicitly said he was cool with it.
So I just don't see Star thinking too hard about the lives of the spells the way a viewer who sees the entire situation would.
Also I guess something to make my argument about the spells not "really" being alive better/worse, depending on your point of view: think of the cells in a human body and how they're alive, but you're not mourning like every drop of blood you end up losing, or something like that. The spells are kind of like that, except in this case they're like the cells of the wand.
And yeah I might just be digging myself deeper with this argument but I hope I explained my own position okay enough.