I can honestly say, without the slightest bit of uncertainty, that "I Saw the Devil" ranks as one of the sharpest and most absorbing serial killer films of the last 20+ years - right up there with "Silence of the Lambs". I was fucking blown away with this wild South Korean flick!
After a secret agents fiancée is brutally murdered by a sadistic lunatic, he devises a complicated method of revenge. He manages to track down and corner the constantly active killer, shoves a small tracking device down his throat, breaks his arm and leaves him alive. The enraged boyfriend continues tracking the killer, assumingly enjoying how the tables have turned, as the killer maintains his vicious compulsion of random killing and raping. His problem, however, lies within the victim's avenger, whose hunting consists of barging in on the killer when he is about to harm another person and beating the ever-loving shit out of him, thus, throwing off his murderous activity throughout this degrading cat-and-mouse "game"! This bizarre pattern of on-going vengeance plays out until the deranged madman finally figures enough is ENOUGH and it's decided that retaliation is imperative...
Truthfully, I could fine no fault in "I Saw the Devil". It is completely satiated with gratifying heaps of savage violence and extremely strong performances (most notably, Choi Min-sik from "Oldboy"), all contained in an amazingly slick and immaculate bestowal of attentive film making. The cinematography is stunning and the effects are incredibly convincing - namely, the beatings with pipes and other blunt objects are fucking vicious looking! No punches are pulled in terms of misogynistic cruelty and fanatical torture.
Not only is "I Saw the Devil" an all-around entertaining mix of graphic horror and action, it also managed to strike a few chords in the way of some pretty affecting hints toward moral conscience and regret pertaining to vigilante justice and the victim becoming victimizer type shit. It all works very well in the way it's cinematically composed and paced. Plus, the ending is oddly tragic.
I guess if I were to nit-pick, I'd say the inclusion of the cannibal character was a bit over-the-top and felt kind of out of place. It's a small gripe, if you would even consider it one. Hell, it doesn't matter. For a flick that almost reaches two-and-a-half hours, it fucking flies by and kept me glued. Highly recommended.