I agree, that's also where I thought the movie was going when I watched it. But maybe we're more interested in or primed to think about anti-essentialism than the average viewer.
Another explanation though: your ending would work best if it were intended as a single standalone film. But, the creators are surely anticipating a raft of sequels. They need to keep the demons evil to set up future conflict in future movies.
I did not get an impression that most demons are fallen humans, I thought that Jinu is one of the very few humans in the underworld. So the ending makes sense -- it's prevention of humanity extinction by the alien soul-eating demons.
(This review contains spoilers for the entire plot of the film.)
K-Pop Demon Hunters is a very popular movie. It is the first Netflix movie to hit #1 at the box office. It is "the first film soundtrack on the Billboard Hot 100 to have four of its songs in the top ten". When you Google it, a little scrolling marquee appears with a reference to a joke from the movie. My friends keep talking about it. So I figured I'd check it out.
The movie does some interesting things with animation, importing a lot of anime tropes and visual effects into the realm of 3D animation. For me this mostly fell flat; a bunch of them landed in the uncanny valley, or were otherwise jarring. That said, the choreogrpahed fight scenes are very well-executed and fun; a friend described the feeling of watching them as "like watching someone play Beat Saber really well", and I agree. The movie's songs are also pretty good. (Honestly, I'm not a huge K-pop fan, but they're very catchy and I expect them to be stuck in my head for a while.) One song in particular was handled cleverly; more on that below.
Spoilers follow!
I expected cool visuals and catchy music from the beginning; the real surprise, as the movie approached its climax, was how engaging I found it on a thematic level. The film sets up a simple Manichean world of good and evil, then dives into an exploration of the troubling psychological implications of this setup, weaving together the protagonist's personal growth and the viewer's increasingly conflicted understanding of the movie's cosmology. Then it throws away all the metaphorical structure in the last few minutes, stabs the problem with a sword, and brings back the status quo. I found this very frustrating! I think it could've been on par with some of the best Disney and Pixar movies, thematically, if it only had the courage of its convictions. (I wasn't expecting The Godfather.)
Our protagonist is Rumi, a singer in the K-pop group Huntr/x. Together with her bandmates Mira and Zoey, she maintains the Honmoon, the (somewhat porous) magical veil protecting our world from evil demons. They also stab any demons who get through the Honmoon with swords. The Honmoon is sustained by good vibes from successful concerts; fortunately, they're extremely good musicians and have legions of dedicated fans. Occasionally Huntr/x fight demons on stage; fans assume this is part of the show, so they don't really bother with kayfabe. (Wikipedia says they "lead double lives"; I disagree. If you don't need to change outfits between singing and slaying, you're leading a very single life. I think most actual K-pop idols are leading doubler lives than that. But I digress.) But Rumi has a dark secret; while her late mother was part of the previous generation's trio of demon-hunting singers (It's a Buffy-style "into every generation" deal), her father was -- a demon! (The implied relationship, and her mother's fate, are never explored beyond this; I can't tell if this was a bold choice or laziness.) Most demons are fairly unconvincing humans even before they morph into their demonic forms; the first demon we meet is watering a plant with a pot of coffee. They also have purple webbed "patterns" on their skin, which Huntr/x tend to use as final confirmation before pulling out the swords. Rumi was born with a tiny bit of pattern, which has expanded over the years to cover much of her body. She is deeply ashamed of this and has kept the patterns scrupulously hidden, even from her bandmates, as she was encouraged to do by her adoptive mother Celine (one of her mom's bandmates).
Huntr/x's goal in the film is to be even better K-pop idols so that they can create a "Golden Honmoon", which will be completely demon-proof. Rumi secretly hopes that this will also rid her of her patterns. But just when they seem to be on the verge of success, the demons send a boy band to defeat them. One of the demon boys, Jinu, turns out to have a secret of his own: He used to be human! He has been turned into a demon by the demon king Gwi-Ma through a typically Faustian deal (earthly power, temptation to sin, loss of his soul, etc). Gwi-Ma, we learn, controls Jinu (and, apparently, all demons) via shame and regret. Jinu learns of Rumi's patterns and tells her about his past; both characters start to hope for a shared redemption. (In particular, they make plans for Jinu to sabotage his band's performance, hoping that he can stay on the human side of the Golden Honmoon.) But meanwhile, trouble is brewing; Rumi has newfound empathy for the demons she mows down by the dozen, and mixed feelings about the lyrics of her band's new anti-demon diss track, "Takedown". ("'Cause I see your real face, and it's ugly as sin / Time to put you in your place, 'cause you're rotten within / When your patterns start to show / It makes the hatrеd wanna grow outta my veins") She starts falling behind in battle and can't seem to get through a rehearsal without losing her voice. Her patterns keep growing. All of this strife is tearing holes in the Honmoon. This comes to a head at the big show-down concert for the Idol Awards; demons impersonating Mira and Zoey perform "Takedown" and reveal Rumi's patterns; she flees, and the news of Huntr/x's "breakup" tears the Honmoon to shreds.
Rumi confronts first Jinu, who has lost all hope and is thoroughly in the grip of Gwi-Ma, and then Celine, who encourages her (as usual) to hide her patterns and try to "fix" things. At one point, Celine says "our faults and fears must never be seen", which we've heard from Mira earlier in the film. Rumi, distraught (and looking increasingly demonic), accuses Celine of failing to love "all of" her. "If this is the Honmoon I'm supposed to protect," she says, "then I'll be glad to see it destroyed." The demon boy band begins a final performance where they sing about unhealthy parasocial relationships for a newly-aboveground Gwi-Ma and legions of sorta-depressed-but-enraptured fans ("I'm the only one who'll love your sins / Feel thе way my voice gets underneath your skin").
Let's pause here. We've learned that demons (or at least some of them) are just humans who have given in to shame and fear and lost hope of redemption. Rumi, on the verge of despair, has glowing patterns just like those on Jinu, the most human-looking of the demons. Her maternal figure encouraged her and her bandmates to hide their flaws; this has now pushed Rumi to the point of questioning her cosmic role as one of the guardians of the increasingly impenetrable barrier between humans and demons. So obviously we're going to learn that the Honmoon was a mistake and that there's a better way to integrate the human and demon worlds, right? And the demons, or at least Jinu, will get a second chance? And maybe we're getting some critique of how people engage with K-pop idols?
Just kidding! As things move towards the obvious resolution on an emotional level (Rumi's bandmates sing about their respective personality "flaws", recontextualizing them as positive traits), they backslide on a cosmological level. Huntr/x has a dramatic battle against Gwi-Ma and his boy band, where they stab the demon king and his demon minions with swords; then they make a new Honmoon, better than ever, powered by the soul-energy of their even-more-devoted fans. (It's rainbow, not golden, but the effect seems to be the same). Hoards of demons get killed or banished back to the underworld; I think maybe Gwi-Ma gets killed but I wasn't really paying attention. A newly-ensouled Jinu sacrifices himself to save Rumi, neatly avoiding the question of which side of the barrier he'd have ended up on. Rumi, for her part, no longer seems remotely bothered by the task of slaughtering demons at an industrial scale.
At the end of the film, it's uncanny how little has changed, for both Rumi and her world. The Honmoon is stronger than before -- although, really, it seems to have been holding up alright from the beginning. The demons (traumatized failures?) are trapped in the underworld, where they (apparently?) belong, and the (charmingly flawed but ontologically immaculate) humans are safe up top. Rumi is basically the same, except that she's not ashamed of her patterns, which are also pretty and rainbow now. I think this is supposed to symbolize her (and her friends' and fans') acceptance of her flaws, but her major flaw has pretty much been fixed. Her bandmates are also more willing to talk openly with each other about their flaws and insecurities, although they really don't seem to have been shy about this before. They're also willing to sing about their insecurities, but this is a somewhat confusing kind of growth. (I don't really think it matters whether Taylor Swift's songs are about specific relationships she's had; a lot of good art isn't autobiographical in that way.) The largest sign of character growth is in their relationship with their fans -- instead of hiding from fans when out in public, they are happy to engage with them. This seems nice, but not really that big a deal.
(There's a possible interpretation here where Rumi's previous shame about showing skin is fundamentally about sexuality, or intimate relationships, but the movie doesn't really seem to be angling for this.)
There's something annoyingly self-referential about Rumi's "flaw". She's ashamed of her patterns, which are the physical manifestation of her shame about them. She hides them from her bandmates, who (we learn) are much more upset about the hiding than the patterns themselves. This is maybe a good metaphor for a lot of personal problems, but most such problems don't go away as soon as you acknowledge them. (Her bandmates -- abrasive Mira and people-pleasing Zoey -- get much less recursive personality traits to struggle with.) We are maybe supposed to think that she's hiding other problems, but if so, there isn't room in the movie for them. As for the fans, despite the movie's insistence that Huntr/x really loves their fans, while the demonic Saja Boys are merely exploiting them, the experience of the fans seems pretty similar in both cases. (It's also not clear to me that a healthy parasocial relationship is simply one where the idol says "for the fans!" a lot backstage.)
Above all, I wish the movie hadn't made its entire cosmology a metaphor for an unhealthy way of handling emotions, spelled out the metaphor in increasingly direct terms, and then left it untouched. It's hard not to walk away with the message that personal growth and spiritual redemption are only for people who are "essentially good" rather than "essentially bad", and that at any rate they're less important than keeping the essentially-bad people in their place.
My friend Tamera suggests an alternate ending: the demons, no longer held back by the Honmoon, swarm the concert -- only to find themselves given new spiritual strength by Huntr/x's music. (This is nearly foreshadowed; audience members' chests glow blow when they're particularly touched by the music, and Jinu leaves behind a glowing blue ball which we're told is the "soul" he regained thanks to Rumi's guidance.) The demons realize they have the power to fight back, overthrow Gwi-Ma, and either turn back into humans or go to their eternal rest. I think this would be much more consistent with the message the movie is going for, and doesn't undo all the work it does in building up the demons and Honmoon as symbols.