at the end of episode 12, hi-soo is on top of the stairs whilst ji-yong is at the bottom soaked in blood. if that means he died first then it could be that hi-soo pushed him. but then she is also at the bottom soon after and another person is at the top of the stairs. if you look closely at the faint image of the person on the stairs at the end of episode 11, they seem to have short hair and a kind of skirt. im speculating that seo-hyun is the one that pushed her because she (so far) is the only one with short hair and at the end of episode 12, she said she would protect hyowon group at all costs.
i may just be overthinking it but yeah, anyone have any other ideas?? :))