I guess he is just so pis*** off to see that he is the very reason that kim dan had been sleeping on the table bruised and looked always exhausted for the few last days. He just does not know why and what made him so angry and even cannot confess his feelings himself and confusions he was blubbering in the last chapter. He has already fallen in love with him so hard and there needs to happen something that can slap the reality in the face. This chapter has the role of a foreshadowing for this. His behaviour as his harsh and bitter words may lead kim dan's leaving him or something worse through which he will realise his feelings for kim dan in the upcoming chapters. At some point the writer had to make him as harsh as he can be, something shocking enough for both sides, for the readers, for the long awaited turn of events. He needed kim dan in his life from the very beginning to be able to change, or rather, to show his true self that has been hidden for a long time. He just needs to trust someone. And we have seen similar chachter evolvement in numberous and all sort of mangas. this character was bound to evolve for the better from the first chapter. We all know this.