i dont this its bc of guilt. and i always thought that the 'secret' was the fact that he's in love w our boy yk. thats why he keeps sending guys to tail him, he wants to pretect him.
Uhm there's more to the story. Yes Taesung might have killed Sooyoung's uncle but it could've been an accident or he was forced to do it. Even if it will crush Sooyoung when he finds out about it, to make Taesung such a monster for doing something out of his control, to me is not fair. He's probably a monster for everything else but at least not that.
I think 50/50 guilt and love
guys remember that boss man killed one of our boy family member i think it was his uncle he killed. And he saw our boy crying on the floor that day at the funeral thats why when they first met he was like u look familiar because boss man was also at the funeral. I think he's with him out of guilt but is also falling in love with him. He's hiding something really bad from him