(I have tried to avoid any spoilers by writing at a high level of abstraction, but better safe than sorry: I threw a spoiler tag on this review.)
What happens when the "rats" of history overwhelm a society? When persons with varying degrees of moral culpability, who each "agree" not to speak of past crimes in return for social cohesion, are forced to continually deal with those past crimes?
The Piper is a deeply disturbing film to view. But I can think of only a few movies in the "horror" or "thriller" genres that have touched me as deeply as has Kim Gwang-Tae's debut: Hitchcock's Vertigo. Aster's Hereditary. Visconti's The Damned. Eggers' The Lighthouse. Through a careful interweaving of folk-lore and Western horror tropes, The Piper ends up addressing a truly humanistic question: What happens to societies when the refugees and cast-offs of various wars and dislocations request assistance and aid, but then fall victim to not only their own individual foibles but also the criminal designs of others?
On one level, the narrative at work in The Piper is a bit of a familiar folk-tale: That of the needful outsider who, taken in by a conservative community, eventually introduces elements of disorder that bring about ruin for both the outsider and the community.
In this particular folk-tale, a man of good heart (Woo-ryong) and his son (Young-nam), are on a journey through a Korean forest on their way to the city (Seoul) so that the son can be treated for his breathing problems (tuberculosis). Tired, they stop in a small mountain village, where the local Chief takes them in on condition that Woo-ryong rid the village of its rat problem. Woo-ryong does so by studying the wind patterns in the village and creating two powders that, respectively, drive the rats into the open and then lures the rats to a cave that Woo-ryong then seals with a boulder. In return, Woo-ryong is to be given the equivalent of the value of "a cow," which money he wants to use for Young-nam's treatment.
But by additionally, first, constructing a swing for the village children (angering the Chief's already jealous son) and, second, falling in love with the village's beautiful young shaman (Mi-wook), Woo-ryong also sets in motion a chain of events that lead to the downfall of all concerned. (We learn toward the end of the movie that there is a very specific reason why this movie is titled "The Piper." The swing itself becomes a very important symbol and plot device at that point as well.)
The figures here are therefore in part the stock-and-trade of folklore and allegory: the needful outsider, the wealthy village elder, the magician, the faithful son vs. the jealous son, the beautiful princess, etc. And the theme, for example, of (potential) intermarriage between clans destroying all concerned is a well-worn path in human story-telling. But The Piper is also properly historical and philosophical in its premises.
Historically, the movie is set at the very end of the Korean War. This is of great importance to the "moral economy" of the story: The villagers are very concerned about avoiding contact with communist sympathizers and spies. Thus it is through the (eventual) accusation that Woo-ryong is such a spy that the moral characters of both the Chief, the shaman and other villagers are revealed. (The socialization of accusation and criminal process in Korea, China and Southeast Asia in the 1930-1980 period is the stuff of professional history texts.)
It also matters because, in a sociological sense, this is a community where people are always "partial" because of the war: Everyone we meet in The Piper is, in a literal sense of the world, a refugee of the war and is further missing someone significant in their life. For example, Woo-ryong and the Chief are similarly situated to the extent that they are both fathers without wives/partners. They are both the (remaining) "heads" of social unit (a family / a village) that is faced with existential ruin. But it is moral character that takes each man in very different directions until the very end of the movie.
As far as we know, neither the Chief or Woo-ryong have done anything to create the condition in which he finds himself. Each is a refugee of the Korean War. But character does become destiny. We learn Woo-ryong is impractically trusting; naive, if you will. For example, of great importance to the movie is the fact that the "mission" here was undertaken because of something written in English by a "Yankee" doctor on a piece of paper for (the non-English reading) Woo-ryong. But once we (the audience) read what it actually says, we realize that Woo-ryong and Young-nam were, in a sense, doomed from even before they reach the village. In fact, even had they never encountered the village, the outcome would likely have been the same.
In addition to Woo-ryong's naivety (which is noticed by the Chief), we also learn that the son, Young-nam, is impulsive and prone to act improperly (if mostly innocently, due to his age). But the Chief also notices this too; and it is his use of this knowledge on two separate occasions against both Woo-ryong and Young-nam that show us that the Chief is a studious, cynical man who can be intentionally immoral in his designs. However, by so acting, he ends up turning the man-of-good-heart into a monster. This is where folklore enters history.
In the end, I take the point of The Piper to be based upon two premises: First, that there are true "victims" of history; persons who, through no fault of their own, find themselves in horrible circumstances and are needful of the aid of others; but that, second, history's victims, in their need, all-too often fall prey to both their own personal foibles, but also the rapacious designs of others. The point, however, is that this sets in motion a cycle of horrors that, in modern times, can never achieve equilibrium because no modern society can forever suppress the evidence of what "really" happened.
Indeed, the reason the village had the rat problem in the first place is the real (though not always obvious) horror in The Piper. This is the Western or Nietzschean element to the movie: That the village only appears to be an unchanging, conservative community that somehow has escaped the vicissitudes of time. It is actually a monstrous, modern creation; and the Chief is merely a modern entrepreneur at its head.
From a cinematic perspective, then, the ending sequence of The Piper might remind viewers of the closing sequences of Vertigo, Egger's The Lighthouse, or Aster's Hereditary. However, where those films have a much more interior, intellectual perspective, The Piper's concern is more obviously global and moral: "This, humanity must address; or no further will humanity go."