Other possible answers:

• Killing the powerful without any need to worry about morality
• Making a world that doesn't treat people like cattle for the powerful.
• IDK, killing monsters is cool?

Still the issue is that this is all interpretive! There's no necessity for a narrative where a particular group is annihilated must be fascist.

I should note here that the annihilation of a class (in real life) need not be deadly (separating killing from violence). But would not the fantasy of exterminating capital owners (those with high magical power) slot in equally well? Again to be fully clear - in our little metaphor there's no need to kill the child (the Chinese proved this). But, to torture the metaphor, if a former child of a billionaire backslid on their reeducation, wouldn't more forceful measures be necessary?

However again, this is all kind of moot - the fantasy of annihilating the powerful doesn't necessarily have to be fascist, classist, or anything in particular. We can leverage interpretations however we wish. What's important is that we take up the fight against just saying "yeah it's fash" without the author either coming out as fash or more complete proof (especially since there's plenty of good shit in the story that aligns with left politics). Why cede the ground so easily when the leap the fascists make (e.g. it works because my opponents aren't human) is one that even the most libbed up idiot would disagree with. Simply pointing out the demons are closer to the aliens from independence day goes a long way to diffusing the fash critique. This doesn't mean it's perfect but it's like, again, make them show that they really don't see their enemies as human. Revealing the fascist as a fascist against the text seems way more useful than writing the text off. They aren't worth capitulating too (they like starship troopers for the wrong reasons after all)