plot twist (and also a long shot), sungji actually met sumin years ago, probably during the time when sumin was in the hospital or some sort (which puts sumin at 24 yo and sungi at 14) and sungji remembers her from some act of kindness or sth.
I've thought about what she meant by that too...It didn't make sense and there wasn't anything more added to it. But I think this could be a key factor to why Sungji fell in love with Sumin and is so considerate of her, even now at this point, where she didn't act out mad but remained calm.
On the other hand, if Sungji wasn't bluffing in this charpter, then her character is too perfect, and almost scary. However, the 3rd arc is from her POV, so we should probably see the deeper side of her also
They said in the intreview that this will focus on Sungji and Seju, and also that third arc will be about "Learning to love again"
As for Seju going psycho, although it is no surprise, it was never confirmed whether she actually did worse things or not. The redhead situation, I found it normal, because Redhead was also cheating on Sumin too, so it was not exactly sabotaging their relationship. It was good for Sumin to know she was been cheated.
I personally don't like how her character is turning, not only because what she did now is very wrong (not that Sumin isn't wrong either), but also how she is turning psycho, only to make people hate her and jumb on TeamBlonde.
I think it would be better to keep her more neutral with hints of sinister like the past chapters, and let people decide who they like more out of Seju and Sungji. Now it's kinda forcing it t go to a certain direction. Well not for everyone,but definitely for some readers anyway.
And also the fact that they may reveal more crap in the future too, making her look even worse than this.
Like Sumin was stuck with her, because Seju was deliberately destroying every relationship Sumin was in. Or even she ordered mole guy to hurt Sumin... Then Sumin founds out about all of this(Sungji helps her too) and realises that this was not love, but she was manipulated by a toxic person and finally moves on.
It was nice how we had a choice of liking either the ex or the current love interest, and this is what made this manhwa nice for a change. But this is the direction it seems to be getting now. It's not only because of what Seju did this chapter, it's also about what she will do in the future too. Things can easily get way uglier than this.