Red Mansion is the only one I can think of atm, but it's one of the ones where I constantly think, "these two are both absolutely insane" (⊙…⊙ ).
Dunno how dark you’re looking for but here’s so I could think of:
Taming the tiger
https://www.mangago.me/read-manga/eyes_clouded_by_the_tiger/
Black Mirror
https://www.mangago.me/read-manga/black_mirror/
Derail
https://www.mangago.me/read-manga/derail/
Shitto wa ai o kumoraseru
https://www.mangago.me/read-manga/shitto_wa_ai_o_kumoraseru/
Red Mansion
https://www.mangago.me/read-manga/red_mansion/
It Starts With Stalking
https://www.mangago.me/read-manga/it_starts_with_stalking/
Private Libido
https://www.mangago.me/read-manga/private_libido/
A Tree Without Roots
https://www.mangago.me/read-manga/a_tree_without_roots/
Where the Wind Stays
https://www.mangago.me/read-manga/the_place_where_the_wind_blows/
I need reccs for bl where both the mcs are red flags/yanderes, yk where they're obsessed w each other to a very unhealthy degree
I don't mind yaoi or shounen ai, tq in advance~