I think he chocked her but she got saved. The girl then exposed him. Then they got saved. But like another comment said if he killed her thank he would have been in juvenile. Anyway this is my guess
the story saves death for special events. but if the story is hinting us in the right way, the girl's neck was injured in a way that probably crippled her. she's probably alive but her life is basically ruined.
anyways Ji soo over here seems to have assaulted his crush? then maybe when Seongjun saved him from public opinion, he made him into some kind of hitman. it makes sense as he asked him to Cripple the living hell out of the Biker Gang Leader Chan.