Alternate Explanation of Plot
most ppl would agree that in the end the serial killer possessed the body of the cop, killed the other cops and raped his wife.
there is an alternate explanation. (i didnt make this up, i read it in other forums)
it can also be viewed that, when the cop was seriously wounded (and probably the first time seeing a dead body) in the opening theme and was in psychological shock, as his superior officer claims. since the entire movie is told from the perspective of the cop, the view has no direct way of telling which footage is real vs imaginery. thus it is plausible that there were no ghosts in the movie at all, but were complete imaginations of the cop.
also to note is that ever since the parking lot killing scene in the beginning, the cop kept on getting weird dreams of people dying that were indeed imaginary. this is strong proof that his mind is no longer "stable" and suggests that he was insane.
in addition, some of the deaths in the movie can not be explained using the ghost explanation. ie the girl who hanged herself in the roof is illogical. for her to act that way, she must be possessed by a ghost first, but if the host dies (ie being shot dead), the ghost too will be banished (unless they touch another living body quickly). thus the dried up girl on the roof can not have been killed by the serial killer.
there is also no reason for the "wheelchair" boy to die. according to the story plot, the serial killer only take pleasure in killing girls. he kill cops only when necessary. however, if the cop was indeeded insane and were responsible for killing these girls, the "wheelchair" boy would be able to testify in court, the where abouts of the cop. thus, the boy has to die as a cover up.
the mass killing in the highschool is also odd, the serial killer take pleasure killing and torturing one at a time, in secret. however it would be plausible if the cop was the real killer and went to the highschool to kill the delivery girl. she must die because she visit the cops very often and knows when they are in and when they are out.
most significant, the movie also mentioned that "what you see may not be real". since the entire story is being told from 1 person's perspective, how do we know he is/isnt telling the truth? it would be too foolish to ASSUME what WE saw from the eyes of the cop is infact representative of truth.