I Saw the Devil - Ending Explained

The film's ending is a masterclass in 'hollow victory.' Soo-hyun rigs a guillotine in Kyung-chul's home, tying him down so that the blade is held up by a rope the killer must hold with his teeth. He then calls Kyung-chul's estranged parents and son to the house. When they open the door, it triggers a mechanism that drops the blade, beheading Kyung-chul in front of his own family. Soo-hyun listens to their screams of horror via the tracking device and realizes he hasn't achieved peace—he has only created more trauma. The final shot of Soo-hyun walking away, crying uncontrollably, confirms that the 'Devil' he saw was ultimately his own reflection in the mirror of his revenge.

One common interpretation is that Kyung-chul 'won' in the end. By goading Soo-hyun into using his own family as unwitting executioners, Kyung-chul ensured that Soo-hyun would have to live the rest of his life with the guilt of traumatizing those he loved most. Another reading suggests that Soo-hyun was always a monster; as a secret agent, he possessed the skills for extreme violence, and the murder of his fiancée was merely the catalyst that allowed him to drop his social mask. Some critics also view the film as a critique of the modern surveillance state, with the tracking device representing a god-like power that ultimately fails to prevent evil because it is used for personal vendettas rather than public safety.