Hes upset to see the guy who stole his ex again on the television so he turned cold out of self defense, in case his relationship with wooyoung would be his dark history repeating itself. Thats the most plausible explanation since he was not fully sure of ywy just yet at that time.
I'm too dumb to understand the situation but why is the seme suddenly act like that in chapter 17?? Like what did i miss???