Then I take it your member title comes from the Korean movie of the same name.
Yes, as a matter of fact. It's a pun considering the theme of the forum.
As for the movie, Korea has a more liberal approach to graphically strong content. Just look at the vengeance trilogy.
However,
I Saw the Devil is not a horror movie it's a cautionary tale. It's the old Nietzsche saying: "Beware that, when fighting monsters, you yourself do not become a monster..." There is no question as to who is the bad guy, the rapist and torturer is; the question is which of the two is the devil. Is the one who kills and rapes, the monster, or is it the one that comes at night to punish him for his sins. Soo-hyun is slowly corrupting himself and becoming just as much a monster as Kyung-chul because otherwise the vengeance won't fit the crimes. It's an old theme and a concept that has been explored numerous times.
By the way, the full quote is “Beware that, when fighting monsters, you yourself do not become a monster... for when you gaze long into the abyss. The abyss gazes also into you.” From
Beyond Good and Evil. That notion is one that HP Lovecraft has build entire stories on. I don't really like Lovecraft but I read that one story that pretty much is nothing but this.