beta
Spoiler from the novel, but it's was on sequel so probably won't be on this mahwa.
Yon Jae was actually born psychopath, or sociopath, or whatever you called it where he lack of emotion and unable to feel empty toward others (this is why he was flabbergasted when Yoohan call him psyco for first time because that kinda bullseye).
His mom was dying of sickness, she know his problem and love him but still, she feel sad and disappointed because he never cry for her. Before died she make him promise to never "kill people" which he agreed (but he twist the meaning "if want to kill, don't do personally").
So he went to Korea for his mom funeral, he was bad mood because until the very end he can't show the sadness his mom wants to see. At first he was meet some random gangster and about to use them for release, but a rabbit stop them and Yon Jae ended fight the rabbit on the bed instead.
Turn out that day was also the day Yohan' mom death. When Yon Jae sees Yohan tear he feel something on the deepest his heart, as like he understands Yohan' sadness and feel empathy towards him...