@WillLi @kryssN Imma have to disagree with you guys on this.
Chapter 72 already showed him to be pretty much "broken". The scene with Ji-eun was his breakdown and the moment he decided to throw everything away for the sake of finding his sister.
The point of the scene was to showcase not only his desperation but also his resolve to achieve his goal by any means necessary. Even if that means following the path of violence and, in the process, becoming what he hates the most (the vivid image of his father).
The fight with Young-joon was not only a fight to prove his strength, during the fight he made a final effort to remain true to what he believed in. That's why he decided to not use the glass and use strategy instead in order to turn the tides of the fight. He managed to do so and almost got the upset. But one final moment of doubt cost him, not only a broken nose but to be humiliated by Maria, the person he considered his mentor and to that point, the closest thing to having a family again.
So now he's forcing himself to do things the way Maria does things. Even if he hates violence, even if he thinks it's wrong. He's convinced himself that there's no other way to achieve strength ad improve as a fighter. And who could blame him? So far, sticking to his beliefs an ideals only ended with him coming short time and time again.
I will admit that the contrast with his "normal" self is pretty shocking, but is not as if it comes out of nowhere. There have been other moments where Jiu Ji-tae showcased his violent side.
Some examples:
Ch. 31
Ch. 61