The trouble with I Saw the Devil is that its cartoon characters do not inflict cartoon violence. The level of brutality on display might be affecting if accompanied by actual gravitas, but it’s impossible to feel deeply for any of the characters because there is so little to them.
It’s not hard to understand a man’s desire for revenge after his wife is murdered by a remorseless, barbaric killer, but the extreme shape his revenge takes would be easier to grasp if we had any sense of the relationship he lost. Likewise, there could be real tragedy in his vengeful machinations if we were given even a glimpse of the soul he was losing in the process. But I Saw the Devil’s characters aren’t developed nearly enough to be meaningfully effective, which reduces its hollow protagonist’s rampage to an ugly parade of nihilistic ultraviolence.
Side note: I don’t know if the English translation is lazy or if Koreans have a limited imagination for insults, but the endless repetitions of “bastard” and “son of a bitch” are pretty grating after awhile.