At first, this series seems like it would be firmly in the crime thriller genre. There are criminals, including a serial killer on the loose from the time the main male character is a child to the time he is grown up and has become a criminologist. There are different criminal cases, but there is a connection between the cases even though the crimes are unrelated. Yet, it does not go very heavy on the solving of mysteries. As some viewers have noted, the investigations don't even seem professional. The story is pretty good fiction, but if you are expecting a solid criminal investigation story or even a realistic portrayal of psychopathic serial killers, this series will defy your expectations. The police don't seem to be very good at their work and requires the expertise of the male protagonist, and the male protagonist himself has to rely on his resourceful friend (a wealthy art dealer of all things) to find out information. Eventually, his reliance on the friend becomes a self-conscious joke in the series as the friend observes how the male protagonist would keep on giving him orders to investigate things. (And yeah, it's not very nice to blackmail your gay friend into doing things for you by threatening to out him to his father.)

I think there are several narrative threads left hanging: I found myself asking at the end, "Wait, so is the guy who flees to the Philippines and gets arrested there a serial killer or has a serial killer made use of him to kill?" Also, what has really happened to the main female character's father? And perhaps the oddest of all: why does the story end with the identified (main serial killer) still on the loose in a drama that sets itself out to be some sort of crime drama? I can't really tell whether it is merely an attempt to have some sort of cliffhanger for Season 2 (which may not happen) or if it is a daring deviation from genre conventions.

A deviation from what one may expect from the crime genre seems plausible because the deviation is present way before the ending. Sometimes, the deviation is interesting. For instance, one might already guess from Episode 1 who the "monster" referred to is, but even when one guesses it, one may watch on to find out how the dots are connected. Also, the fact that the main serial killer has someone in the police force helping him secretly is strongly hinted at (together with the precise identity of the person helping him), but even upon getting the hint, one may want to watch on to find out exactly why the person is helping the killer. This makes the story more fun than merely having sudden revelations that no one would expect, something that I think too many series try to do.

On the other hand, there is awkwardness in the way the series defies genre conventions. For one, the serial killers (yes, there is more than one) are "humanized" a little too much. They seem to be psychopaths though I don't recall the term being used in the series at all. The term "monster" is used, perhaps because the series is meant to explore human nature without pathologizing certain behaviors. There is allusion to a story about how there are two wolves in every person (one good, one evil) and how the wolf that one is fed will govern the person's behavior, and the duality of human nature seems to be the focus here. Yet, the series seems to take this a little too far in showing the humane side of people whom we might understand to be psychopaths, showing them to be highly capable of true empathy and affection. Yes, there is the nature vs. nurture debate, but is it possible for someone to have the capacity for love and empathy when both nature and nurture have worked together to efface the person's good side (assuming there is one) from a young age, as in the case with the male protagonist's brother?

The depiction of the male protagonist's relationship with his brother is also quite unconventional. I'm pretty sure it is not my BL-fan imagination shaping my perceptions, but there are points when the brothers are depicted almost in a way one might expect lovers to be depicted. It might well be possible to do a montage of their scenes together and convincingly present it as a trailer for a BL series that does not exist. It's not that I mind the portrayal of brothers being really chummy and not exhibiting any toxic masculinity towards one another--after all, it is not as though there is actually any incest going on in the story. But what is the point of it all? Especially if we remember that the younger brother is actually rather selfish in telling their father not to believe what the older one says, his longing for brotherly affections can't possibly be as innocent and sympathy-inducing as the series eventually makes it seem. I would guess that the younger brother just wants to be loved without loving anyone, but this doesn't seem to be the case eventually. In other words, the story seems to venture into a rather dark terrain and then suddenly chickens out and heads the opposite direction instead.

Many seem to hate the main female character, but she isn't really that bad. There are some points when she is a rather badly written character. For instance, she is supposed to be a rather feisty person and a daring police officer who will investigate and stop crimes even when there is danger, but then she would suddenly behave like an idiot afraid of ghosts when exploring a house. Unfortunately, the character seems to be quite unnecessary (especially with the lack of elaboration on what has happened to her father) and seems to be placed in the drama for some obligatory romance though some scenes between the male lead and his brother seem more romantic than her scenes with the male lead.

Overall, the story is quite intriguing even with its flaws. I don't know how faithfully it has followed the Korean original, but assuming that it has followed the original story closely, I think it should have taken liberties to correct the inherent flaws in the story.