Love this story, as a fan of psychological horrors, even if sex scenes make me squirm a lil'. Also from the author of country of maestri so thats an added plus haha
Anyway, I love meself some theorizing, and since we're kind of in the part of the story that is adding a lot of different moving parts, quite literally, I'll just mention it here:
So there is something supernatural that seems to be occurring here. We could assume it to be a hallucination, but the thing is that even external sources are confirming this random disappearance and pools of blood. That's why I'm doubtful this is simply hallucinations from our MC.
Second thing, I believe all deaths involved a 'reflection' at the scene. The parents in the dark room where there was a mirror, the theatre had a glass screen between our mc, and the random disappearance and pool of blood while our MC was walking on the street was in front of a reflective glass that had the 'creepy smile' self on the reflection. Also considering that this is called 'Black Mirror', I'm inclined to believe that our MC has some sort of connection with his powers and a mirror/reflection.
It also seems to be fueled when he is with/thinking about yeon woo. Yeon Woo previously mentioned that they were "connected" through the glass shards that cut them. I'll even add that prior to the parents death, they're personalities were almost switched, in a way (maybe this has to do with their connection). Also the fact the MC is an orphan is also sending out an alarm that there is more to him that we don't know as well.
What does this mean then? Well, to be honest I'm not quite sure. Which is why I love this. Any theory can be plausible so long as it fits within the evidence that has been provided. Because the evidence given is still quite vague, I don't really have a proper solid theory, but my current assumption is that our MC may likely be the one to have the powers (since it happened once even when Yeon Woo wasn't there, but rather he was thinking about him and had a headache). This power can be fueled by Yeon Woo, which is why it never resurfaced until he met Yeon Woo again years later (after choosing to forget him). This never happened with Yeon Woo alone, so he likely isn't the 'source'.
Either way, I feel like we still don't know a lot, and I could be wrong about some of the stuff.