Has been my theory since day one!!
Mokyeon very likely has borderline personality disorder.
Back in days, psychologists used to believe bpd patients existed on the borderline, between neurosis (depression) and psychosis (disconnect from reality) So the title borderline or boundary of Delusion makes sm more sense
Well, in season 1 Mokyeon had that depressing tone, unstable relationships, bottling up his emotions, self harm tendencies and not to forget that office scene with Nanjo. He had a violent side. But in season 2 it becomes more clear that he does infact have a lot of unresolved trauma and has most likely developed bpd.
The scene of him as a child eating cereal while imagining he's on a quiet beach and the same office scene with Nanjo from season 1, he kinda dissociates from reality when his emotions are high.

Okay following the past two chapters I am convinced this story is called borderline because the lead character has borderline personality disorder and he needs serious help and not a romantic relationship with the man he’s stalking. You guys can’t possibly think it is normal to self harm and stalk someone’s job over a heartbreak, someone you weren’t dating or in an actual relationship with, someone you just had sex with a few times. Mok is 100% the kind of dude that would threaten to kill himself if you leave them or threaten to kill you if you leave. The author is literally showing us a character spiral out of control and self destruction and y’all are busy coddling him and making everyone else to be the villains all the while avoiding taking accountability for Mok’s actions that led him here. I still don’t feel bad for him, I do think he needs a therapist though