I can imagine this drama is not to everyones liking as , for a K-drama, it is rather dark and at some points uncomfortable.
Although it has some very minor flaws I find it rather outstanding all in all

+ Seo In-Guk is simply fantastic in this role. I didn't know him before and after having watched some dramas for the past two years I think this was the first male lead who completely convinced me to be that person he portrays. When I was watching this I really believed this very person would exist like this, and I could not imagine him to be any way different. There are other actors I like and they are good, but in their roles I always see the real person they are, when you watch interviews with them.
Here it was very authentic. The fact his character was difficult and at times mean and cold made it even more realistic for me, as in general, people tend to be like this. They have their personal troubles and deficites, they can be mean and sometimes, not always, they might have a good side too. But in the end, they are caught in their own ways and can't get out of their own skin and troubles. And that often leads into disaster, for themselves and if one gets close to them, for others too. That is what is shown here for me and that makes it believable. Uncomfortable, dark, but believable.

+Jung So-Min was also convincing in her role. She portrayed very well the inner struggles you might have in such situation, the insecurities you might have from hard times in your childhood, the way things went ok for her with being adopted by her 'brother', but how her past still creeps back into her life and drwas her to Kim Moo-young. This drams is also good at showing certain rules of behaviour and manners that one has to know about concerning Korea. Some criticised that Yoo Jin-Kang 'betrayed' her 'best friend' Baek Seung-Ah, but was Seung-Ah really her best friend? I don't think so. It is not explained fully how long these two women have known each other but it is clear that Jin-Kang is older than Seoung-Ah, yet, probably because of her considered poor and bad family background Jin-Kang is almost submissive in her behaviour towards Seoung-Ah - which is quite exceptional given an obvious age difference between the two. If Seoung-Ah wasn't filthy rich and Jin-Kang had a better background, it would normally be the other way around. And Seoung-Ah is using this fact in a gullibly naive way all the time. Kim Moo-young even points it out (and rightly so) when he tells Jin-Kang that not even after Seoung-Ahs mother hit Jin-Kang in public and offended her deeply (and referrs to exactly that poor seen background of hers), Seoung-Ah doesn't even apologise in her next text message but just once again wants to use Jin-Kang as her servant and messenger. Is that Jin-Kangs best friend? I wouldn't think so. It's more a realtionship of power and inferiority as a given by the strong Korean conventions. Was Seoung-Ah a victim? Yes, certainly, a victim of her greedy mother and to some extent of Moo-Young, but she was not a victim of Jin-Kang in my opinion.

I found the almost creepy but strong attraction between Jin-Kang and Moo-Young very understandable, and I could relate to it personally having had such relatioinship myself too in the past. I fully understood for many reasons why those two couldn't get away from each other and also why it was in some way doomed.

+I loved the mood and the small things in the drama. The way Kim Moo-Youngs place looked, the light, the mood. His feelings of desperation, resignation, anger and still the need to somehow connect and try to understand at least some other person. That does not justify all his behaviours of course, but I found it credible. And again, when I was younger I would have fallen exactly for this type too, I would probably even have stood in front of that door of his like Jin-Kang in the end, crying in desperation and beg him to open that door, because I didn't understand his Jekyll & Hyde actions after a steady emotional up and down. But it is exactly like certain real relationships are, and how they end unhappy in one or the other way. And how they are filling you up with an almost hysterical happiness and emotions you wouldn't have considered possible for you, and at the same time they are like energetic black holes exhausting you to an incredible level.

- if I would want to criticise anything then it would firstly be that some circumstances are rather unlikely. The idea that these two kids where from Haesan, one of them got lost somewhere and all of them live almost aroud the corner in a megacity like Seoul more than 20 years later and meet like that - it is very unlikely.

- the scene with the origin of the burn marks wasn't done in a good way. The kids were fully clothed and if that water would have hit them, they would have had some minor burns, but never such large scars for the rest of their lives.

-the whole stabbing scene and the follow-up was a little unbelievable . So Yoo Jin-Kook loses his police cool and while he later says he felt sorry for Moo-Young, he suddenly decided to stab him on a public road. Jin-Kook confesses to his boss who...does nothing actually, doesn't accept his resignation. Then his little sister just acts rather normal after shortly screaming at her brother? Not believable. Her reaction at the first moment was right, she said he must be crazy, and she was scared then and angry especially because she is very much in love with Moo-Young. The next thing would certainly have been that Jin-Kang leaves that house. Jin-Kang would have been afraid of her own brother after his rather surprising and totally crazy actions. I would have wished that whole part would have been done differently or they would have left it out. It would have made much more sense to me if Jin-Kook would have gotten into a rage and then into a fight with Moo-Young, if he had beaten him up maybe. But that stabbing with murder intentions was an unfitting twist here.

Summary: despite these flaws this drama touched me deeply, maybe really because I could relate to the main characters and this kind of relationship so personally.
I will certainly rewatch it at later as this kind of human darkness draws me in sometimes.