I Saw the Devil (2010)

SPOILER: Jang Kyung-chul (Choi Min-sik) is a dangerous psychopath serial killer. He has committed infernal serial murders in diabolic ways that one cannot even imagine and his victims range from young women to even children. The police have chased him for a long time, but were unable to catch him. One day, Joo-yeon, daughter of a retired police chief becomes his prey and is found dead in a horrific state. Her fiancé Soo-hyun (Lee Byung-hun), a top secret agent, decides to track down the murderer himself. He promises himself that he will do everything in his power to take bloody vengeance against the killer, even if it means that he must become a monster himself to get this monstrous and inhumane killer.