After watching The Wailing those who could not match the ending or did not understand who is the devil or the good or the main culprit?
The main culprit is Ojha and the Japanese man. Originally the Japanese man was Main Kalprit, he was the symbol of the devil. Ojha comes later to help the Japanese man.
The young girl in the white dress is the symbol of God here. He wants to save the village from this black shadow or evil spirit. But being deceived by the devil, in order to save the girl, the police officer basically releases the Japanese man in the form of the devil, and the black power wins.