I have a different take on this as many others.
First, I had a hard time getting into the story, mostly because I disliked the cop's acting so much. Not Ji Chang Wook's acting, but the character's acting. He was supposed to be a cop, not an actor, but this was still extremely annoying. He behaved like some hot headed, not very smart brat that wanted revenge. I have no idea how the mafia boss could trust him. Every time he opened his mouth, he sounded so damn fishy. He was impatient and pushy. It made zero sense that the mafia boss kept him around and trusted him. He did try to dig up his past, and he remained cautious of him, but it wasn't enough. I know he was blinded, since he trusted the woman, and she knew his new subordinate. But the mafia boss was too weak. I honestly don't recall a single scene where I felt he is that fierce, dangerous and murderous guy he was supposed to be. Or maybe I just wanted him to be like that. Honestly, he was soft and composed most of the time. The more the story progressed, the stranger it felt, when the cop called him "boss", because they changed their positions. It 100% felt like the cop was the "boss" and not the other way around. So for me, this was a major flaw in the story, that the mafia boss was too soft and too trustworthy. After all, he build a big business, so I expected him to be smarter and more ruthless.
Second, and that's the biggest problem I have with this series: It was not possible to understand what those characters felt for each other. People praise the acting in this series, yes, it felt intense, but was it really that good? I don't blame the actors, more the director or writer, because I assume it was intentional. But there was no real depth in their performance, no real connection between the characters.
So let me go into detail: We have 4 main characters and main relationships.
** The relationship between the wife's cop and the mafia boss **
I never felt for a single moment that she genuinely liked him (in a romantic way). I felt she had some positive emotions towards him since she knew him for so long, and she remembered him when he was young. There was some nostalgia involved. But that woman was so damn plain and everything about her was boring. She was just looking pretty the whole time with that soft expression on her face. The relationship between her and the mafia boss was very creepy. I honestly have no idea how he could think this would work out since she is a cop. He should have known, she is not that naive. What she witnessed - how could she ever be okay with it? Those two together on screen made me uncomfortable. But then at the end, she was desperately sobbing? What were the writers trying to tell us? Was she sad for herself and what she did?
** The relationship between the cop and the drug cartel girl **
That was the only relationship where I felt there was something genuine going on. He even admitted at some point (and I believed it), that he may like her even more in the future. But the focus was on his emotions and his struggle to give in since he is married. So there was not enough room to really develop this relationship. I also didn't understand why the girl liked him in the first place. The qualities of his mafia persona were being loyal, she called him a "soldier". Well, he is attractive, that may have helped. The girl came across very smart and tough, despite being a small and beautiful woman. So it ruined her character that she failed her job so badly, just because she trusted that guy (who wasn't trustworthy at all). I felt in love with Ji Chang Wook watching K2, but in this series he wasn't attractive all (not physically, but his whole personality and attitude). So it's even harder to understand why she was so into him.
** The relationship between the two guys **
I disliked the cop from the beginning to the end. He wasn't a good guy. He was a cop, and he was hesitant to kill people = good guy? For that damn promotion, he was willing to do everything. And don't get me started about that bloodbath, because then it was clear who the real monster was. They made it look like he did all of it to protect his wife, bust since there was no chemistry between those two, it was just hard to believe. Not to mention that he let that guy die in this alley, when he could have called an ambulance. He would make a good gangster! That the mafia boss decided to make him the new boss wasn't that far-fetched, I may admit it. It's true the cop got things done, he wasn't murderous, he kept a clear head even in difficult times, and he was a hell of a fighter. Still, he was fishy and unlikable until the end. Those two had not a single genuine (!!!) bromance moment. It was the naive mafia boss who reached out to him emotionally and was open with him.
** The relationship between the cop and his wife **
There is not much to say about this: boring, no spark, no chemistry. I couldn't sense any love here, it was more like some obligation. They could have made her his sister, wouldn't change anything. But they deserve each other, that's for sure. And they deserve their fate. I was applauding (not crying) at the end. Serves them right. They shall feel guilty as long as they live. Despicable human beings. All of them. Until the end I have no idea - and not because I don't get it, nope, it wasn't shown clearly - what the cop and his wife felt for the mafia boss. How they reacted to his death, why the cop shot him in the first place. I read different theories, none makes much sense. I personally think, he just did it, so he can get the promotion since he caught the mafia guy. Sounds better than: He came to my home and shot himself in the head.
So to sum it up: The mafia boss was too naive. The supposedly good guys not good at all and not likable. I couldn't root for them, also the other cops. They made a big deal out of the death of one of those cops, when that guy also was such an annoying and unlikable character, and we didn't even know much about him. And the drug cartel girl was blinded by love, which made her look rather stupid.
Yeah ... I don't know. The cinematography was great, the actions scenes too, everything was intense and interesting, the pace was just right, the music was great. It was a very intriguing experience, and not a bad series at all, hence my high rating. But I'm not head over heels in love with it, because there was potential for a really, really good bromance, and for three really intense love stories. But they didn't go there. Nope. None of those relationships felt genuine, deep, or passionate. You just couldn't feel much love here, despite "love" being the downfall of the supposedly bad guys.
While I'm writing this, it really hit me hard, what this series could have been. Just make the cop more emotional, more of a good guy, so his "change" into a monster is more heart-breaking, make his wife less plain, and this could have been a wonderful series that would make me sob like a baby at the end. I didn't dislike the end at all, but it had not the emotional impact on me, it could have had.