I'm not sure why it is so highly rated but it just seems like a lot of people who don't watch a lot of movies may have rated the movie. And I guess since so many users seem to have mentioned that they watched the movie in Korean, yes I did as well, because a dubbed movie loses its essence.
The movie should've been called "Coincidence". The second half especially is very frustrating. I've been on a korean spree recently and this by far is the weakest one out of them and I'm still rating it a 3(well 2.5 or so would've been apt) which tells a lot about the quality of the other movies!
Well of course it isn't all so bad. The acting by both the protagonist and antagonist is fantastic. The way the movie has been shot is.. Like all the other korean movies, ie. beautiful. People who love blood and gore will enjoy many scenes, but then again they might as well watch "I saw the devil" instead.
The movie in itself is very watchable but its the coincidences and lack of clarity (not ambiguity, but a lack of clarity by the creator as to why so many plot conveniences) make the film much inferior than it actually should be. But again, it's not all so bad because certain people learn certain lessons, the hard way.