I Saw The Devil 2010

Kyung-chul is a dangerous psychopath who kills for pleasure. The embodiment of pure evil, he has committed horrifying and senselessly cruel serial murders on defenseless victims, successfully eluding capture by the police.

On a freezing, snowy night, his latest victim is the beautiful Ju-yeon, daughter of a retired police chief and pregnant fiancée of elite special agent Soo-hyun (Lee Byung-hyun). Obsessed with revenge, Soo-hyun decides to track down the murderer, even if doing so means becoming a monster himself. And when he finds Kyung-chul, turning him in to the authorities is the last thing on his mind.