“I Saw the Devil” starts out as a well-executed, particularly gruesome revenge horror/thriller movie, with the sort of tropes and violence that come with that. When it turns to the “by the protagonist pursuing revenge, they are in fact becoming like the monster they are pursuing!” morality play in the second act, it feels a bit trite for all of how well-made it is (the lead two performances are particularly captivating, as is how the director deftly handles the kinetics and staging of his scenes).
The film frontloads that sort of morality play into the second act, where the protagonist does something so bizarre and overcomplicated it quickly becomes clear the film is less interesting in depicting a typical revenge cat-and-mouse story as it is pushing the impulse for such stories to a logical conclusion. The monsterisation of the protagonist is done quickly, as he dismisses moral concerns in his quest to not kill the antagonist, but inflict the sort of stress and pain upon him that he inflicted upon his victims, through a series of bizarre, violent episodes.
The third act pulls this to an even stranger level, where the relative monsterisation of both protagonist and antagonist is remarked upon as useless given there’s nowhere to go from there (the inherent banality and eternity of evil gives little for him to go on). So the protagonist conceives of the antagonist more softly, demonsterising him rather than himself, and therein lies the key to the film’s novel, striking conclusion.
An interesting stakes-raiser for the genre. Three tracking devices and a guillotine.