I interpreted it as Nanjoo being mad at Mok Yeon for trying to control him. Even though Mokyeon says he chased sunhwa away for Nanjoo’s sake (because he knows that nanjoo no longer has feelings for him while Mokyeon anderstood that he actually likes nanjoo), he was clearly crossing the line. He even had sex with nanjoo while he was sleeping with sunhwa, so I think that was Nanjoo’s initial reason for being angry. However, what made him even angrier was the fact that Mok Yeon kept insisting that everything he did was for Nanjoo’s own good, acting as if he had the right to control nanjoo’s life and make decisions for him
he certainly has many legitimate reasons to be upset with him. i phrased it the way i did because the extent of the physical violence far surpassed (in my eyes) simple anger, and went into “i have been pushed to my breaking point and snapped” territory. which is a culmination of all the awful things blondie has done.

Can someone just tell me what he did to deserve the beating? I only saw a bit from the new chapters so all I know is the sad childhood lol