"I Saw The Devil" is ultra-violent and portrays women in gruesome circumstances.
Women who are being killed are not "models dressed sensuously across forests". There is nothing pleasant or "cool" about these scenes; They can and should leave the viewer uncomfortable, the purpose is this, disturbing, bring the human mind to places very obscure as the subconscious.
This gives us an insane view of what a human being is capable of, and does so realistically without appealing.
"I Saw The Devil" is realistic and brutal. This movie is great. Beautiful cinematography contrasts the dark and dimly lit scenes where murders occur. Camera work is simple but effective; The viewer is often treated with close-ups of Byung-hun Lee and Mik-sik Choi, and his facial expressions tell us more than the dialogue could ever tell.
The story itself is typical of revenge movies, but fantastic in its execution. Lee's character experiences a profound loss at the hands of Choi's character and, in the process of revenge, begins to resemble the man he hates. The line between "victim" and "offender" becomes blurred between the two characters.
As for failures, the only thing I can think of is the odd, maybe unrealistic behavior of the police. Still, it is a very trivial complaint that it is not worth commenting on why.