Many of us got confused on that too but the consensus is that Sungho got a sudden awakening from his delusional state when he saw Dohyun crying.
He has been working in a delusional idea that Dohyun needed to remember their past and that they were lovers and that's why he was redoing all the things they did .... in his twisted mind, once Dohyun remembered he once 'loved' him it will be okay and 'come back' to him.
Obviously the kidnap and raping wasn't the best way to win Dohyun back. Sungho is convinced that once remembered his advances will be welcomed but he is suddenly confronted with the reality of those tears. Whatever rage, hate and despair he saw in those tears brought Sungho to a moment of sanity and forced him to see the horror that he did. Like a crazy person who is suddenly regains sense in the middle of killing someone. That's why he retrieves from the image of Dohyun and gets his own breakdown outside ... if he got sane enough to intend to free Dohyun we don't know because all that he can say now is that he is sorry and he didn't intended that outcome. We have to see that Sungho is not a sane person so even if for us his logic and thinking doesn't make sense, that is the plot in the story -- he is behaving like a crazy person and therefore his motives and techniques is something that makes sense for him. If he walks back to redemption or sanity and do something to atone for the harm, we will never know if the story ends here.
I like the way you explain (≧∀≦)
Your sentences are greatly constructed!
Thank you for the answer (๑•ㅂ•)و✧
Can someone explain me what happened in Ch 42? Like o don't get what dohyun said and why sungho went to the other room and screamed