I think the author is using Ji-tae's downfall into violence to touch upon the topic of self-loathing. The idea of him not wanting to look weak is present in the entire chapter and is reinforced by the way he carries himself.
You could say those insults he was throwing at the twin were also directed towards his "old self". The scene also mirrors that moment in the alley when Maria berated him after he got his nose broken.
I know a lot of people are missing the sweet and kind Ji-tae, but the path he chose the moment he agreed to team up with Maria was never gonna be sunshine and rainbows.
The thing he wants the most is to find his sister. But, in order to do so, he must become a ruthless fighter who's strong enough to go to Fight Class 3 so he can later repay Maria by helping her killing his own father.