I watched it twice, forgot that I watched it once. I never understood why it has such a high rating.
Connection to real-life events and accurate representation of it isn't justifying it.
It never felt finished to me, "that's it?" was my thought when I saw the end credits roll...
As a straightforward detective story or whatever, it is inherently lacking and unsatisfying for obvious reasons (murderer never caught) compared to Hollywood thrillers with a similar topic topic (something like Seven for example) but i guess that´s part of the point since people were even more so unsatisfied since the murderer was not caught in real life but that does not make it noteworthy in itself.
Beyond the detective story, the second layer is the real life connection (hence the detective at the end looking at the camera because at that point the murderer was never caught, and it was partly supposed to imply that the murderer is watching the movie and the detective was looking at him).
But as usual with the movies of, in my opinion the three great Korean directors, Bong Joon-ho, Park Chan-wook and Lee Chang-dong, what makes the movie great is that everything shown, the murders in itself, the characters, how the investigation was conducted, how desensitized the people were about the murders and so forth, were allegories alluding to Korean society post 1979 under dictatorship and especially after the Gwangju massacre. But understandably so, and that makes the entryway/hurdle to understand the movie very high, you might not make that connection without at least basic knowledge of recent Korean history.
Or, unlike me, some people are not impressed by that and prefer the actual story (I guess first layer) to be more exceptional in itself, which is not wrong, I just don´t share that viewpoint.