Not sure if they'll be like "The guy upstairs" but I'll suggest some thriller ones I've read:
1. My deepest secret (you've probably read it already, previous work of the same author. If you haven't, the plot twists are mind blowing)
2. Chasing Tails
3. Dear X (not really like the guy upstairs but it falls under thriller genre, many psychological stuffs too, it was good, you can find this on webtoon)
4. Bastard
5. Killing stalking (if you haven't read it already, it's pretty intense and gruesome so upto you)
7. Never ending darling
8. Killstagram (I read it in 2021 and I remember liking it, idk about now as it was my 2nd thriller manhwa, stalking included)
9. Trash belongs in the trash can (this is more psychological, ml fl both psyco and there sm backstory, pretty intense, loved it)
10. It's mine (includes stalking)
11. Uriah
12. The predator
13. Only hope (romance+thriller, story depth, backstories)
Can someone recommend a manhwa that's similar to "the guy upstairs"? Bro I need a genre like this fr