My interpretation of the movie
The following is my attempt to explain the movie. Nevertheless, there are still a few points that are inconsistent with my explanation.
There are two supernatural forces at work: the feeble good, represented by the woman in white, who is the guardian angle of the province where the events take place, and the potent evil, represented by the demon we see in the end. The guardian angle has a fixed physical form, however the demon has no physical form (though it does have a spirit animal: crows, but they possess no special magical powers; they simply mark its presence).
Both the guardian angle and the demon can exert magical powers only through the mediation of human beings, however they gain access to humans in different ways: whereas the guardian angle must ask a person to follow her orders without questioning (i.e. to demonstrate blind faith in her), the demon takes hold of a person's body forcefully by raping them. There are two other, rarer methods by which the demon can come to possess a human being, as explained below.
In the beginning of the movie the demon inhabits the body of the Japanese man (it remains a mystery how the demon came to inhabit this man), however after the shaman's death hex ritual is interrupted by the protagonist (i.e. the policeman), the hex backfires and the demon, wrenched out of the Japanese man's body by the shaman's hex, takes over the shaman. Thus in the beginning of the movie the Japanese man is bad and the shaman is good, but after the interrupted hex the Japanese man is good and the shaman is bad.
While the (still good) shaman is performing the death hex ritual to kill/banish the demon, the (possessed) Japanese man is performing a ritual to resurrect the corpse he had found in a stranded truck in the middle of the forest in a previous scene. Once the demon leaves the (now good) Japanese man's body and moves into the (now bad) shaman, the Japanese man realizes the gravity of what he had done and runs in panic to the truck, hoping against hope to find the corpse as dead as it had been when he had spotted it there previously. To his horror, the corpse is not in the truck. He realizes that the corpse has turned into a zombie due to the black spell he cast. He runs to his hut to try and stop the zombie, but it is too late: the zombie had already been killed by the gang that the policeman had put together, and the gang is now after him, intent on killing him. So he runs away.
When the gang chases the (now good) Japanese man with the intention of killing him, they are chasing an innocent man; innocent in the same sense as when an insane man kills another person while in the throes of a psychotic episode is not considered responsible for his actions. Therefore the gang's murderous intentions towards the (now good) Japanese man, and their subsequent dishonorable disposal of his body, constitute a sin. Due to this sin, the demon, who had been wrenched out of the little girl's body as a result of the (good, at the time) shaman's death hex ritual, is able to repossess her.