@nana Just keep in mind that this is only my opinion, but I feel like Mokyeon shows a lot of traits that remind me of BPD, especially his extreme fear of abandonment. Looking back at his childhood, he was always lonely and seemed terrified of being left by the people he loved. Even as a child, he had a tendency to hurt himself when he was abandoned by his parents.
And then he met Nanjo, and for the first time, he found someone he genuinely loved and became deeply attached to. His fear of losing Nanjo became so overwhelming that he could no longer think rationally, which I think explains why he ended up doing what he did at the end of S1 and even now, that same fear and attachment seem to fuel his obsession with Nanjo. It feels like he’s still struggling with that same fear of losing the one person he can’t bear to be without.