@JessieR do not have intimate relationships if you believe consent can't be withdrawn at any time even if someone paid you. 
Dan repeatedly said no, not here and other variations of wait/stop. Jaekyung ignoring him in these moments is a violation, it's sexual assault. 
There is also the issue with him having Dan blindfolded with headphones on and telling Heesung to come over, not even allowing Dan to know or consent about it. That is also sexual assault and a gross violation of trust.

@Alfjomichi  justify means to explain and validate his actions which is incorrect. There is no justifying what he's done to Kim Dan. Especially the chapter in which he has blindfolded him and made him wear headphones so he wouldn't hear Heesung come in, that's se*ual assault, as is many other instances in this story. There is never any justification for that.  

The word you're looking for is simply 'explained'. It explains why he behaves the way he does, but as OP stated, that is not an excuse for how he behaves. As you say, you have to take responsibility for what you do. He's accountable for his own actions, and his past may explain, but it can never justify.

@Alex they're talking about the author, the author is called Mingwa. Also, no need for the casual misogyny.

@Noname  yeah but c'mon, contracted or not, there's no need for how violent he is, nor the exhibitionism he subjected Dan to unknowingly, and besides a contract like that would not be legally binding, of course people are mad at him.

@Throw him away  that's what I'm scared of.

@Aanciaa thank you!