However I can't blame anybody who doesn't like it. If you don't pay a lot of attention even to the smallest things, you will lost and think: it is a mess with so much plot hole and stupid decisions. It's a hard drama to watch and the amount of characters makes it more harder. A notebook fit well with this series. It was a not so good decision to not tag it as a Sci-Fi since the basics (DNA, surgery) are in fact not be too far from the truth. (Google is your best friend!)

My recommendation is do yourself a favor and find out soon who is the serial killer, because later on -after episode 6- it won't be the focal point of the story. So you can focus on another really important details which will lead to the only big twist. From episode 7, the real question is not Who is? but What happened? and in the last episodes: Why? It's not a big deal to find out who is our psychopath tough you have to know what you should search for. (Checkpoint is on the end of the rooftop scene in episode 6!)

Sometimes the story seems over-complicated because it's overcrowded. It's a thrilling experience with a dark and very twisted world. I know It's hard to concentrate every single detail all the time but at the end it will pay off.

That's very interesting how the show presents its characters. We can't embraces the most of them. Maybe you can sympathize with them. The most likeable character is Moo Chi who has the biggest character development. We can feel his sadness and grief, sense his motivation... and what about his deep depression which led to his alcoholism. These characters are complex and interesting with morally questionable motives.
The bad guys are bad all through! Some of them are born that way (as a psychopath) and some become monster cause of events. (Not all murderers are psychopaths and not all psychopaths are murderers!) Our main serial killer is a mentally ill psychopath who murders anybody within his own rules. And the rules are very sick like him. He is a true psychopath with god-complex and delusions. His smile gives me goosebumps. But he is not the worst!

Technically, the cinematography and sound-design is excellent but in some scenes the edition and lightning is poor. I also don't like the censorship but its my overall problem with this genre in the Korean film industry. The themes are very interesting and important. There are a lot of social problems and moral controversies in this series. Viewers can see how detailed the criminal side of the show. Also Pyo Chang-won - a very famous Korean criminal profiler and expert on police studies- worked in the show and his lecture was an important part of the script. (He has a short cameo in the last episode) . There are references to real crimes as the infamous Cho Doo-Soon case which was inspired Bong Yi' s background story. Congratulation to the cast, especially to LSG. This is his best role without any question. If He won't win any award for it I will be angry as hell!

On the whole, It became my favorite psycho-thriller k-drama with its complexity. It is well-made and well-written but not for everybody. Who don't mind blood, violence, dark themes and world, just start it! This drama is unique in its genre.