I believe the girl he fcked in ch8 is a totally different person
http://www.mangago.zone/read-manga/killing_stalking/mf/c008/82/ http://www.mangago.zone/read-manga/killing_stalking/mf/c008/84/
She looks the same to me ... but yeah, that could be just the general look that the girls in this manhwa have ...
Jieun said she wanted to get to "taste" Sangwoo in a different way or something along those lines. She probably said that because she was a thirsty bitch and wanted to know what it was like to have sex with him. So yeah, the girl in chapter 8 might not have been her.
Damn you Sangawoo...you lied ... in chapter 8 there is shown that you slept with her... I won't forgive you for this ...and other things....