Yes, I used to think the same, but at the author's note at the end on back to school, the author answered if Chiwoo and Jihyun (I don't know if you write it like this) had sex and she said that they actually did it, that is the real reason of the trauma, and for Chiwoo to throw up when they meet again. Actually Chiwoo never thought Jihyun killed someone, but still remain without seeing him because what he did.

I’m officially a hater and I love myself for it cause rapists will always be on my hate list no matter what(I’m a hypocrite though cause Killing Stalking is my favorite fucked up read and that little shit ruined Yoonbum’s baby heart too-) (⌒▽⌒)