I Saw the Devil (2010)

Kyung-chul is a dangerous psychopath who kills for pleasure. He has committed infernal serial murders in diabolic ways that one cannot even begin to imagine and his victims range from young women to even children. The police have been chasing him for a long time, but unable to catch him. One day, Ju-yeon, daughter of a retired police chief, becomes his prey and is found dead in a horrific state. Her fiancé Soo-hyun, a top secret agent, decides to track down the murderer himself. He vows to 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 the monstrous and inhumane killer.