Atheist (but not anti-theist; just one who does not have a religion) here, just to get that out of the way.
I talk to people at work, and sometimes I mention a movie my niece is obsessed with, because most people who have kids (or have kids in their family) know that a lot of kids watch this movie... sorry... religiously.
I find that a lot of Christians find the film or the idea of the film offensive, and they won't say why, so I thought I'd try to ask online where people might be more open with their opinions.
So first of all, Kpop Demon Hunters takes place in Korea. I'm not sure what the Christianity numbers are there, but the film is based on local legends and lore. There are no American characters in the film with speaking roles. Probably not European, either.
Second of all... they're musicians who moonlight as demon hunters. They send demons back to a shadow realm which might be equated to Hell (but not quite, it's closer to the surface and demons can walk the earth in disguise, but with glowing veins ("patterns") that set them apart.
Third... the way the girls fight the demons is by inspiring their fans through music to create a honmoon (or, spiritual barrier) to keep most of the demons out. The stronger the honmoon, the more demons kept out. Golden honmoon means all of them are kept out, so that is their goal.
I feel like it's Christian-compatible in that what amounts to prayer is keeping the demons at bay. However, I do understand that it goes against the First Commandment, "thou shalt have no other gods before me." The fans aren't praying to Jesus or God, they're praying to pretty teenage girls who sing pop songs and that's what keeps the demons at bay. To be fair, the fans don't know this, and any demons they see, they assume are part of the act. To be absolutely clear, none of the fans are praying to the singers. It's the inspiration in their hearts that fuels the barrier.
But I can't explain all this at work without offending people.
I look at Christian bands like Petra, and it seems to me the Kpop Demon Hunters idea could be VERY easily translated into a Christian film. Pick a Christian band and do the exact same thing. In fact, Petra did make a movie in the 1990s about a young man who loses faith when his brother is diagnosed with cancer, so he takes a road trip and the band's music turns his heart back. It's kind of the same idea minus the demons (unless you mean the threat to his faith).
I grew up in a conservative Christian household that was firmly in the grip of the fear wave in the 90s, so maybe I can provide insight.
As another commenter mentioned, the word demon is an instant red flag generator to these people. But even if they understand that the heroes are anti-demon, there are some other aspects they may find unpalatable. Believe it or not, providing cultural context will likely only upset this camp further. A culture that has a concept of the supernatural outside the bounds of their theology immediately invokes fear and guardedness. In this context, the idea that humans can fight the supernatural outside of God's power would be seen as humanism at best, or possibly occult.
Understand that a lot of these people grew up in a culture of fear, especially fear of other theologies. Christians are commanded to not let the sinfulness of the world infect them, and unfortunately people take that to mean stuff like this instead of things like greed, pridefulness, or any of the host of abhorent things defining the American evangelical movement right now.