I am not surprised about her development, because I don’t think this was development, it is simply picking up where it left us off about Sumin.
The writing has been on the wall about Sumin’s personality since around chapter 50 or so when she went from being shown by the story as victim of bad circumstances who then closed off her own heart in defense, to someone who purposefully strings alone Seju who legitimately loves her (and probably the person who has or ever will love her the most, period) for a decade, cheats on Sungji who she supposedly now loves and treats even Nammi and her own subordinates at the company not that great tbh lol
I mean, who would think all this is just because of some trauma or whatever? Everyone has trauma, it’s commonplace, shit happens. Seju was even more traumatized, she never even had people care for her, and she didn’t go around treating people badly.
The author then solidified the idea that Sumin’s circumstances only amplified her bad personality traits in Paradise. The way she treated Sunhwa then was callous, even if it could be justified for a teenager. Even her explanation to Nammi for why she felt bad about Sunhwa dying was all about saving face. She is extremely self-centered.