but I think an actual horror romance story would be like if the ML was a yandere or the ML was a jerk(full stop, no heart of gold) and the 2nd ML was a yandere(in the worst kind of way as in the FL doesn't even see him as a love interest).
Which funny enough is a typical jousei/shoujo set-up but you just need to make the female lead have more than two braincells, not constantly blushing, and a goal that involves not getting a man.