I think the story line is too complicated for a 2 hour movie, may be better for a 6 episode series so that the story can develop at a slower pace and viewers have the time to process and understand. Maybe it's ok for Koreans, but for me as a foreigner with little knowledge of their history, I find it difficult to take in the whos and whys. I have many questions while watching, why he is doing this, why he is suspecting them, etc etc.
On the whole this movie is not bad for a first time director. Action scenes and acting are great. I like the 2 male leads and other supporting actors are good too.