Jevil fits "egoism" better as well; it's just harder to make a Stirner Jevil.
I'm fully aware this is jumping full into speculation-tier, but honestly I'd love an infodump on how monsters construct their food so we could analyze the political economy of monsterkind. Their society seems to resemble capitalism, but we have no idea how their nourishment translates to how many magical goods they can produce. I suppose those are just arbitrary variables and the fundamental points of Marxism would still stand though, even if valued in monster-food units instead of human-food units.
I didn't see Undertale making a political point. The protagonist is a kid. How many kids become killers? It was more a critique on RPGs and grinding mechanics than anything about violence being good or bad.
Vaguely /leftypol/ related AU:
A post-Pacifist world where Muffet dominated the world through her business prowess, and her descendants dominate a cyberpunk world 80 years after monsterkind left the underground. Main character starts as a libertarian type, the best of their political analysis being that if we get rid of the state and the "cronies", the market will set itself well again.
Good ending: Main character, through allying with those either struggling under and/or fighting against the capitalist system, realizes that a society run by the market cannot stand and joins the communist revolution, understanding that ArachCorp is not the end, and never could be, not until capitalism worldwide is destroyed.
Bad ending: The character joins in with the corps and their ideology goes to full-on ancapism. After getting promoted for their ruthlessness, they are pushed to the conclusion that relying on the idea of "property rights" to make sure that no one takes from you is nothing but a show of weakness, becomes a full-on egoist, slaughtering their porky allies and taking their wealth(and perhaps literal power through some soul-harvesting mechanic), only for it to be implied that they too will fall to the revolution eventually.
Neutral Ending: They get blackpilled and become a shitposter like us I guess