I love this drama, really. It is not perfect. It is sometimes hard, a bit boring, but it just gives it a specific atmosphere. The heroes are not great, idealized, they are full of vices, like any normal person. This adds realism to the drama. Some elements are unreal, others are exaggerated, but hey, this is a c-drama, you will find such elements in each of them.
History, although it has traces of banality, is not. The drama was created in 2014, when the topic of serial killers and psychopaths was not yet so widespread. Of course, a criminologist or psychologist would have some charges against this drama, but this is an entertainment series, fiction, not a documentary series.
Finally, I must say that the music is great. It really gives the right dark mood and lets you feel the atmosphere of the presented reality.
I have seen "Gab Dong" three or four times so far. I already know who the killer is and how the fate of the characters went, but the atmosphere of this story is really unique and can absorb the viewer.