The story follows Kim Soo-hyeon, a National Intelligence Service agent, whose life is shattered when his pregnant fiancée is brutally murdered by Jang Kyung-chul, a sadistic serial killer. Rather than turning the killer over to the authorities, Soo-hyeon embarks on a monstrous quest for vengeance. He captures Kyung-chul, beats him, plants a GPS tracking bug in him, and releases him—only to hunt him down again every time he attempts another crime. As detailed on IMDb , the film explores the psychological toll of this cycle, as the boundary between the "hero" and the "monster" evaporates.

This scene redefines the title. “I saw the devil” no longer means “I identified the external monster.” It becomes a confession: “I recognize the monster within myself.” Soo-hyun’s refusal to stop the cycle—even when given multiple opportunities to hand Kyung-chul to the police—cements his transformation. The index has completed its migration from Kyung-chul’s actions to Soo-hyun’s choices.

This methodology introduces the first indexical shift. Soo-hyun does not seek justice; he seeks to make the devil suffer . However, in doing so, he adopts Kyung-chul’s own logic—treating a human being as a plaything for sadistic pleasure. The film indexes this change visually: Soo-hyun’s composed face increasingly mirrors Kyung-chul’s vacant, predatory stare. The devil is no longer just the killer; it is the methodology itself.