I Saw the Devil (2010)

Kyung-chul is a dangerous psychopath who kills for pleasure, leaving devastation in his wake. Soo-hyeon, a top-secret agent, sets out to track him down on his own and make him pay. Driven by a promise of vengeance, he pursues the serial killer through a brutal cycle of capture and release, willing to do anything in his power even if it means becoming a monster himself.