Spoiler
Gayoung is part of the cult/terrorist group that distributes drugs (Prometheus I believe?). She experiments on several people because she wanted to create artificial hunters. In the process she asks Sayoung if he'd like to participate and Sayoung accepts—tho imo he did it out of desperation and it was clearly unfair on him since, well, he was a kid and J was gone by that point—so she experiments on him. The experiments were horrible and several people died, Sayoung awakened but it's said that it wasn't actually thanks to her experiments, and Jungbin finds him and takes him with him.
Later on, when J goes against Gayoung, there are actually some of her experiments with her that kinda simulated Sayoung (who she doesn't recognize) but were once before people, and are by then deformed and lack the ability to think by themselves.
Does she atleat get brutally killed for being an absolute bitch??
mm i haven't finished the novel yet so I can't say. however i read up to ch 313 and she was still alive and not facing any consequences yet
What did the girl we hating on do? I'm assuming it's the doctor from the last panel named Gayoung?