Its true that it is hard to find good yandere stories that actually make you scared of the yandere! Yandere as horror is truly underrated. I used to have a list of scary yandere stories on bato before it got taken down. But some of my favourites on the list that i can remember the names of are:
Manga/Manhwa :
Never-Ending Darling (free on webtoo...... reply
did dropped a manga or webtoon
