If you are writing or looking for a paper about this specific movie, scholars typically focus on:

Directed by Kim Jee-woon and starring Lee Byung-hun and Choi Min-sik (of Oldboy fame).

How the film blends the traditional police procedural with "slasher" and "torture porn" elements.

The film centers on a "catch-and-release" game of torture between a secret agent and a serial killer. Critical analysis often explores how the protagonist becomes a "monster" himself, mirroring the evil he seeks to punish.