This is another movie which I regretted not reading reviews before watching. Reading the synopsis did not prepare me enough for it. I watched because of Jang Dong Gun and Lee Jong Suk. Expecting more of the political plot and the difficult process of catching the psycho in detail but after watching can't seem to shake the psycho look/actions out of LJS and can't bear watching his dramas/movies now without thinking of him in this role - that's how good he is at portraying this sick character.
On a side note, if you're into this type of gory thriller, you may be interested to watch an old Korean movie "I Saw The Devil". I watched this then because of Lee Byung Hun. The only thing I'm glad of is, he did not play the role of the psychopath in this movie.
Both movies shows how sick a human can be in mind and action and doubt I'll ever re-watch because it's too disturbing and you can't unsee what you've seen. The senseless violence and brutality of it may remain in your mind for a long while.