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?? :))

It's the chief maid. She tried to kill jiyong in episode 12 by trapping him in that bunker. 

She will see him and hisoo having a tussle. Being angry at him and unable to kill him in ep 12, she sees this as a golden opportunity to finish him once and for all, so she pushes him off the ledge.

Idk where I got the idea from, I must have associated the red color with something else, but I really thought the man got knocked out with a FIRE HYDRANT in of the way earlier scenes of the bodies.  Because like @clarywayland20 said the height isn't insane to get bashed up like that