I have to disagree. Shiyun has used Kyunghee's mother to manipulate him throughout the entire manwha. (The sole reason he got close to her was to get to Kyunghee) The only person's happiness and well being Shiyun cares about is his own. And I think Shiyun is still manipulating Kyunghee to the very end. I will agree with the murder/suicide thinking by Kyunghee at the end.
Yeah, but it implied he was only manipulating her to keep Kyunghee in his life. EVERYTHING was to keep Kyunghee in his life, but there were several moments where it showed that Sihyoon wasn't happy with his relationship with Kyunghee being forced and his conversations with Kyunghee's mother alluded as much as well. It's fine if you disagree, but it really is all there.
I'll quote your original post - "Sihyoon wanted whats best for Kyunghee, it's what he talks about with Kyunghee's mother twice, he just couldn't help but hurt him to keep him in his life. He really hated their situation and wanted a normal relationship, he says this numerous times when he says things like "if you'd just behave things wouldn't be like this"
So, if Shiyun truly wanted what is best for Kyunghee instead of himself, he wouldn't have gone to such extremes to keep an unwilling Kyunghee on a leash. Even though Kyunghee is not totally innocent in this game either... In the end, a horrible price was paid by an unwitting victim. (Mom) To each his/her own perspective.
"if Sihyun really wanted what's best for Kyunghee instead of himself, he wouldn't have gone to such extremes to keep an unwilling Kyunghee on a leash"-this right here is where we disagree, it's entirely possible to wish you could give someone/thing everything while not being able to for one reason or another. What i've been saying is that, while he WANTS to have a good, willing relationship with Kyunghee, he sees that it's impossible and so to keep Kyunghee in his life he HAS to keep Kyunghee on a leash. He loves Kyunghee very much, it shows him hurting multiple times over the state of their relationship, albeit in quick/subtle moments, but he made his decision already and he planned to follow through with it to the end. It'd all be worth it for him if he can just have Kyunghee. He says "if you'd behave it wouldn't be like this", he literally says he'll treat Kyunghee well if he'll just accept him for real. The entire point of this was to try and weasel his way back into Kyunghee's heart, Kyunghee's mother was just necessary collateral damage as Kyunghee wouldn't even look at him without her as the bargaining chip. He gets visibly upset when Kyunghee doesn't fall for him, he hates that Kyunghee is fighting him and that he goes to sleep with Junsik. He sees that Kyunghee is being pushed away despite his efforts and he gets really jealous and upset. There is REALLY heavy implication that Sihyun is in love with him, like, a lot.
-continuation- by the end of the manga Kyunghee is broken, Sihyun is broken from seeing Kyunghee broken and, i think, they are no longer in a romantic relationship. I feel that it would have been pretty explicit in showing it if they were, like the rest of the manga. Kyunghee has no thoughts about Sihyun after his mother dies, don't you think he'd have some thoughts on him if they were still together? What would Sihyun even use to manipulate Kyunghee at this point? The feeling that the manga gives is just "irreverible damage", the kind that killed even Sihyun's will to hurt Kyunghee for the sake of their relationship any further. Kyunghee is dead inside and there is no longer anything for Sihyun to grab onto. He sees what happened and is just kinda... There. This is what i think, at least. You won't be able to convince me otherwise tbh, so if you disagree then just say so and we can leave it at that. I just didn't want you ignoring that there definitely is a lot of implication that Sihyun loves and really cares for Kyunghee.
Actually, the author states in the extra at the end that "Kang Sihyun (who started this whole mess) got Kyunghee back" and Kyunghee "isn't free of him." She goes on to note, however, that this is not the same Kyunghee from the start of the story, but someone who is "completely dead inside." So it's definitely not the ideal romantic relationship Sihyun wanted. I do agree that it seems like Sihyun is looking after him out of concern and perhaps Kyunghee plans to kill both himself and Sihyun in the near future.
That's as open ended as the last chapter itself... "got him back"? Well, yeah. We see that they're together, my argument was that they aren't in a romantic relationship anymore BECAUSE Kyunghee is broken now. I think what it means by "he got him back" is that Kyunghee is no longer actively trying to escape, but only because he wants to take Sihyun with him to "complete" his family with a "new father" up in heaven with his mother. During the final talk with Kyunghee's mother the topic is "giving up your own happiness for the sake of Kyunghee's happiness", you see him get really straight faced and contemplative about this while saying to Hyunjung that she be greedy every once in a while. It looks like he's really taking what she says seriously in his case, but that he doesn't think SHE should have to because of how much she already thinks of Kyunghee. Then he's completely straight faced when he goes to tell Kyunghee that the worst possible thing has happened, he doesn't whisper shit into Kyunghee's ear to manipulate things, he just sits back looking pained and watches as Kyunghee dies inside. I think what it means by "he got him back" is that Kyunghee is no longer actively trying to escape, but only because he wants to take Sihyun with him to "complete" his family with a "new father" up in heaven with his mother.

It seemed like Sihyoon was just taking care of him as he's suicidal now. I don't think that they're, neither of them, happy at all. Sihyoon wanted whats best for Kyunghee, it's what he talks about with Kyunghee's mother twice, he just couldn't help but hurt him to keep him in his life. He really hated their situation and wanted a normal relationship, he says this numerous times when he says things like "if you'd just behave things wouldn't be like this" and in the way he treats Kyunghee gently when they aren't fighting. Now he sees Kyunghee broken and you can tell just from the ambience in ch 39 that their relationship is just... Sad. He's not smiling and he barely talks, all he says is that Kyunghee should head back and that Hyunjun wouldn't want this for him. All his flare/haughtiness is gone. There isn't really anything indicating they're together anymore. I think the only reason Kyunghee is still in contact with him is because of what he says at the very end, "I'll be there soon. With my new father". It looks to imply that he's still interacting with Sihyoon so that he can "bring" him to her. It mentions that Sihyoon quit his job before showing Kyunghee going to visit Junsik in prison for some closure, placing flowers at his mother's grave while revealing the cuts and then him and Sihyoon going... Somewhere. I think it's supposed to imply 2 endings here: 1. That Kyunghee is going to kill himself and either talked Sihyoon into joining or Sihyoon offered to join as he's his world. This explains the closure with Junsik and the flowers on the grave that, for some reason, reveal his cuts. Or... 2. Sihyoon has been taking care of a suicidal Kyunghee who plans to kill him and himself for his mother. It's either a double suicide or a homicide into suicide, i think. That's it, there isn't really anything implying they're still "together".