
The author wrapped it up quite well.
When the old man declared that he will adopt him officially, but eventually made him like a pet again, on their final talk the old man realize that he simply can't control him so he just let him do what mookyul wants, thats why when they are searching for fox jun, mookyul stated that it is nice to have some power and he can do much more (indicating that he will accept being the old man's heir).
As for the old man he knew very well that someday, someone will appear and turned everything upside down, thats why when sangchul directly confront him to stop treating mookyul like a lapdog he said that sangchul isnt the person that can make him (mookyul) disobey him ( old man)
(c) Sangchul's diary.
The old man making mookyul as his son, negates or simply cut his desire and obsessiveness he can no longer be torn of letting him go or wanting him.
He can only choose of letting him go ( by adopting him as his son ) he finally decide.
Does the black haired guy just have sex with the old man in chp 13? Anyway who is the pair here? It is worth reading? It seems it has many sex scenes that does not involve the mcs together?