That last panel is really important. Abusive relationships don’t happen overnight. They happen gradually.

The first time Yaldar had attacked Black for asking about a photo he found, he didn’t visit the studio for two days. 

The second time, as shown above, Black was angry and asked out loud why he was attacked as he knew he didn’t do/say anything wrong. But over time, and with the truth that was dumped on him after this incident, his conflicted emotions towards Yaldar started surfing up with each new beating. Adding Yaldar’s self-harming in front of Black’s eyes as a way to show he feels guilty to what he had done to the little kid, to the point of actually committing suicide (ch27) made Black’s sympathy to his mentally disturbed brother grow more than his hatred, developing what is known as Stockholm’s syndrome