I think he just underwent a change of feeling concerning Sul. I personally think she's one of the only people (along with his mother, and possible In ho, or whatever his name was, before the whole "hand incident") that he actually cares for. His caring is a bit one sided though. Everyone else he just puts up with, having a façade of niceness.
What caused Yoo Jung to change his opinion of Sul? Well I think he had a bad impression of her from the start and never really gave her a chance to change that. The fact that she tried to be nice and greet him after the party incident probably just confirmed his first opinion. As they were both always watching each other, he probably realised, slowly, that she was not like his opinion of her. The fact that she also has father issues probably made him interested. And I think as well that, because Yoo Jung knows that Sul knows his "other side" and still trusts him (though how far that trust extends...who knows?) means she knows who he truly is and doesn't reject him.

...well, that was quite garbled, anyway...

I am the master of my fate;
I am the captin of my soul.
William Ernest Henely

I rate manga with 5 being the average score.

Jung might be a bit twisted but I personally think that his feelings for Sul are genuine. I want to point out this scene and that scene to prove my point but it would probably end up as spoiler. But yeah, aside from his feelings for Sul, I still don't know whether he is just misunderstood or he is a real sociopath.

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