No one here ever suspected the kililer was...
Detective Cho Yong-koo (the original partner; not the one from Seoul)? I felt like this was alluded to throughout the film; I know that you're not supposed to 'know' the killer, but there were so many things throughout the movie that made me suspect him in particular, and was surprised that no one mentioned them.
1. Generally abrasive and objectifying of women throughout movie.
2. In the first interrogation scene with Kwang-Ho, he is literally mouthing words to Kwang-Ho while Park asks him to look into his eyes, and asks why he isn't doing so.
3. He was the one who filed the paperwork on the first girl that was murdered, but who was never connected to the killer, and supports all statements made that she went to Seoul (granted, this could just be him protecting his incompetence).
4. He keeps his hands in his pockets in nearly every scene, especially when interrogating suspects; always kicks them for some reason (really, it's always a kick).
5. Hangs out at Kwang-Ho's restaurant; i.e. the scene where Park shows up and asks why he's there, and he flips out.
6. Gets stabbed by Kwang-Ho; I heard somewhere that he was the one who burned Kwang-Ho, but i can't seem to remember anything related to this in the movie so moot point. Anyway, his treatment fits with what Kwang-Ho said before he died.
7. What Kwang-Ho said/did before he died. He wasn't looking at the picture, because he knows any picture they show won't be the person; it's their partner.
8. There are two scenes where an individual is asked by Park who the killer is, and his shadow is over Park's shoulder in the distance. He's also randomly not around for large portions of the investigation, and never pursues or develops any of his own leads.
9. The former victim who survived was particularly scared when she heard Park was a police officer.
I know it isn't supposed to be him, but I thought the suspicion complemented the ending nicely, and expected to see a post on here about him but didn't.