Joo-yeon (Oh San-ha), the pregnant fiancée of NIS agent Soo-hyeon (Lee Byung-hun) is murdered by a serial killer and rapist called Jang Kyung-chul (Choi Min-sik). Using the resources of his profession and quietly supported with information provided by Joo-yeon’s police father, Soo-hyeon finds the killer quickly enough, preventing another murder while he’s at it. But instead of doing your usual vigilante thing, Soo-hyeon hurts the killer, implants a combined tracker/listening device in him, and lets him go.
Which initially confuses Kyung-chul quite a bit, but there are many women and the occasional man to murder, so what does he care? Of course, Kyung-chul starts caring when Soo-hyeon regularly swoops in to stop another murder, roughs him and knocks him out only to disappear again. Soo-hyeon, it turns out doesn’t just want his monster to die, he wants him to suffer.
And he’s really neither in the mental state nor a deep enough thinker to understand that what he truly does is to provide Kyung-chul with entertainment and a fun challenge, at least for a time; and, no surprise in a member of the intelligence community, Soo-yeon doesn’t have the moral backbone to care about all the innocent people he’s endangering for his vengeance. Like Kyung-chul, he’s exclusively focussed on his own desires.
Of course, once Kyung-chul has enough of the game, people Soo-yeon does care about will be threatened and murdered.
I’ve read quite a few comparisons between Kim Jee-woon’s I Saw the Devil and that masterwork of feel bad cinema, the original Martyrs. Apart from a degree of cruelty and rather a lot of torture, there’s precious little to connect these two movies in my eyes. Where Martyrs is all about talking about cycles of abuse and eventually exploring philosophical/spiritual matters, the South Korean film is more interested in going as far with the tropes of the serial killer thriller and the vigilante movie as one can go.
Spiritually, this is really rather more Hannibal – the Ridley Scott version - in its love for the grotesque and its improbably over the top plot; unlike that movie, this is made by a director at the height of his powers rather than an aging director of usually highly vacillating talent phoning it in getting rescued by his cast. Plus, South Korean popular cinema of this era is a lot more willing to really dive into the unpleasant places than Hollywood (or DeLaurentiis) usually is, so this does get as extreme in its cruelty and portrayal of nasty violence as a film made on a considerable budget for a mass market could possibly get.
Morally, we are in the usual serial killer “looking into the abyss/looking right back atcha” space – just that this abyss is allowed to be particularly deep and empty, and the guy getting looked back at probably even worse. People, the film’s extremity suggests, are either victims of monsters or monsters (Jang Kyung-chul isn’t the only serial killer in here), and we are mostly watching what monsters get up to because who’d watch the other film.
Because all of this might be just a wee bit nihilistic, the filmmakers turn this into one of the best-looking films imaginary, where every shot is brilliant, every edit absolutely on point, every chase or moment of suspense maximally exciting in the nearly magical way of South Korea’s commercial cinema, with a central acting duo that burns with quiet (Lee) or very loud (Choi) intensity. Thus, we can really revel in the various horrible things happening here and enjoy them.
That this makes watching I Saw the Devil more uncomfortable than watching a less slick film containing exactly the same material would may very well be the film’s point.

