I go with Nerd. Like most Korean films, it manages to be gripping in a very classical sense using keys, cellphones, tower level, labyrinth like streets, handcuffs, shrapnel, hammer, and so on. Weak when it suddenly uses a bunch of narrative tricks to earn our feelings for the 'one last victim' (who could be salvaged) & then go on to turn our gaze to the victim's as we're proved wrong. The massacre itself is choreographed with empty virtuosity.
It's no "Memories of Murder" in showcasing Korean police force's investigative shortcomings, or "I Saw the Devil" in macabre-level vengeance seeking FaceOff. The informative tidbits about the Serial killer, much like the killing sequences itself, shoddily opened-up. All the "suggestive" bits (artistic curiosity, religiously inclined torture methods/techniques, impotence, and so on) were nullified by quirky Slasher-esque demon-looking performance. The only great thing about it is the lead performance of "chaser", nearly Choi-min sik level in later stages.