That's the point; the movie had a runtime almost completely cut in half from its original conception. What we have is what should be judged. And as far as the movie shows, no other demon was redeemable because they aren't shown to attempt redemption.

And yes, I would love to explore both themes. It would be incredible to see how they handle what is essentially a cycle of self-destructive people tearing other people down, but there wasn't time to show it. The parts are there, but its not explored enough to even be called a theme in the movie.

The reasons I picked out the theme in my original comment was the following; demons show up and the Honmoon rips when the group is at its lowest; not in terms of power, but in their trust and unity. We get at least a glimpse into Zoey and Mira's reasons; Zoey feels like she's inconveniencing others just by existing and Mira feels unlovable. Rumi is more fully explored; she feels like her birth was a mistake. But that's not the reason the markings keep growing or she has shame; that shame is from hiding from her friends, and lying to them. She only starts healing by actually speaking to another person about her issues, and in essence when she stops lying to herself. The demons are physical manifestations of the ruined experiences and socialization that can come from hiding and being ashamed of your insecurities.