Shine Son:
Ji Dong Hee is CCY's assistant, confidant, secretary, driver and everything else... But not a friend. A friend can only be happy for a friend but by the way he is acting in the recent episodes... He is not. Do you see CCY considering him as a friend as well? I don't get that vibe.
However, these kinds of hints... The ones that make the viewers focus at one character as the "obvious" culprit can sometimes be used by writers to divert our attention from the actual culprit. This... Sun Jae's brother is one. Ji Dong Hee could be another.
It could also be possible that the culprit is a woman/girl.
You're off it mate. The most recent episodes and promos for the next episode, all but confirm that Ji is nuts and is actually the younger brother of the middle or high school girl that committed suicide and who was embroiled in the murder case of his own mother.
Personally, I like the show and I think it could have done without all the murder mystery but I guess it's there to cause friction for the leads.