In the start of the manhwa, Seungbae tells Yoon Bum that there is an increase in the number of home invasions of Sangwoo's neighborhood. Those "home invasions" could have been Sangwoo breaking in and finding victims
Seungbae already saw Yoon Bum deliberately entering Sangwoo's house in the first chapter, contradicting Yoonbum's explanation in chapters 63-65 that Sangwoo kidnapped him. Why doesn't he remember Bum? Well, for one, he was only seeing Bum from a distance and his use of glasses implies visual impairment. Bum also panicked and turned his back so there's a chance he didn't see his face properly. In addition, how many of you remember random strangers you see in the street enough to recognize them again?
Bum's uncle sees him as a substitute for Bum's mother, who he loved. Given that he abused and raped Bum, who's to say that he didn't do the same thing to his mother? What if that was the reason for their breakup?
Bum once said that he's still living with his uncle in the present. However, when we see Bum's apartment, the uncle was not there. When they met again, the uncle mockingly asked Bum if he's selling his body to be able to live with Sangwoo. It's therefore possible that when Bum was living alone, the uncle would come over to rape him in exchange for money and to ensure Bum's silence.
Bum's Grandma set Bum up. The night that the uncle initially raped Bum, she likely heard what was going on and decided she didn't want it to occur in her space, which is why she sent Bum to sleep in the uncle's room the next night. The fact that the uncle walked in his room and spoke about Bum hiding at the sauna makes it highly likely that she reported the incident to the uncle and told him that Bum would be in his room. The Author admitting that the grandma was alive at the end of the series means that the grandma was there for years of sexual abuse against Bum and never did anything to stop it.
Did both of Bum's parents really die in an accident? Are Bum and Sangwoo step brothers? Bum's Uncle told Bum how Bum's mom left him for his "tall and handsome" brother and then starts sexually abusing Bum on top of his physical abuse, because Bum looks like his mom, using him as replacement. Sangwoo's father was tall and handsome and Sangwoo uses Bum as replacement of his mom because Bum apparently looks similiar to her. Not that the abusive relationship was already bad enough to begin with...
