I genuinly can't tell at what point it's considered horror, cause technically things like Profundis should be considered one tbh, so I'll just go off what gives psychological horror vibes to me:
Hikizuru Oto
Dead Man Switch
Blind Play (just don't read the side stories)
Room without a window (if you can accept a rape scene done to MC by third party. Main couple is very cute though)
I only listed completed ones obviously, because a lot of things can still happen in ongoing series.
Then these should fit as well:
Little Mushroom (one of my absolute favs)
Shitto wa ai o Kumoraseru
Knock me out
Amber Alert
The Pizza Delivery Man and The Gold Palace
Death Delayed for a Short Time Because of the Will
Kimi no yoru ni fureru
At the end of the road
Healer (I'm currently letting this marinate but from what I know its similar to room without a window in the sense that everything SA related is either only implied or done by a third party. AFAIK it's a very dark story but the seme treats uke well)

This was tamer than your average BL manhwa out there, lol
How a psychological horror story has no SA whatsoever, but when I read "romance" with comedy I'm confronted with random rape is truly beyond me