Controversial opinion, but it sorta seems like the FL was pretty toxic in the relationship as well? Their relationship lasted at least two years (before beginning of her second year to the beginning of her first year in university), but her internal dialog makes it sound like she never told him that she loved him, completely avoided showing affection, and outright pretended they weren't dating when other people asked for the whole two or so years.
Not saying the murderous yandere isn't toxic, just that she definitely contributed to the problem and that a sane person would have probably broken up with her or would end up with some serious issues.