wkb

Yes “rainbow city” novel (manhwa isn’t there yet)

'Global examination'-- the romance does take a bit to build but afterwards they're both equally caring and want to protect each other

https://www.mangago.zone/read-manga/sangyang_the_wanderer/
Have you read this yet, Pookie?

and its gotta be like, you least expect it from them? ykwim

itou-san

merry psycho

Maybe not technically clinically insane, but these all feature mentally unstable MCs or FLs/MLs - some actually diagnosed with some sort of mental disorders and others just have fucked up personalities; some might not fit the criterias you're looking for:

https://www.mangago.zone/read-manga/lovedeath/ (GL)
https://www.mangago.zone/read-manga/girl_in_a_cage/ (GL)
https://www.mangago.zone/read-manga/hone_no_nazuki_made_aisu/ (GL)
https://www.mangago.zone/read-manga/saeism/ (GL)
https://www.mangago.zone/read-manga/useless_princesses/ (GL)
https://www.mangago.zone/read-manga/tatoe_sekai_ga_owattemo_watashi_wa_kanojo_wo_erabanai/ (GL)
https://www.mangago.zone/read-manga/your_fetters/ (GL)
https://www.mangago.zone/read-manga/the_barefoot_nina/ (GL)
https://www.mangago.zone/read-manga/after_school_love_affairs/ (GL)
https://www.mangago.zone/read-manga/her_lies/ (GL)
https://www.mangago.zone/read-manga/i_m_more_dangerous_than_you/ (GL)
https://www.mangago.zone/read-manga/i_love_amy/ (GL)
https://www.mangago.zone/read-manga/you_are_still_in_a_bird_cage/ (GL one-shot)
https://www.mangago.zone/read-manga/rapunzel_komoda/ (GL one-shot)
https://www.mangago.zone/read-manga/a_story_of_a_girl_who_killed_her_best_friend/ (GL one-shot)
https://www.mangago.zone/read-manga/i_m_bound_by_miho_chan/ (GL one-shot)
https://www.mangago.zone/read-manga/i_love_you_hanatsu_yaya/ (GL one-shot)
https://www.mangago.zone/read-manga/anata_to_kanojo_wa_watashi_dake/ (GL one-shot)
https://www.mangago.zone/read-manga/the_final_confession/ (GL one-shot)
https://www.mangago.zone/read-manga/moonlight_kie/ (BL; psychopath ML; violence, ephebophilia, abuse, it's ongoing but it will also feature non-con, dub-con, confinement, ML attempted to kill MC)
https://www.mangago.zone/read-manga/eighteen_s_bed/ (BL; everyone except one kid seems mentally fucked up ngl; violence, bullying, homophobia)
https://www.mangago.zone/read-manga/the_edge_of_ambiguity/ (BL; ML is mentally unstable - possibly has schizoid personality disorder?; coercion, emotional manipulation, mild blackmail by a side character, violence, sexual assault on the MC committed by a side character, depictions of panic attacks, depictions of suicide, threats of suicide, underage drinking, child neglect and abuse by parent, stalking, forced surveillance, forced isolation)
https://www.mangago.zone/read-manga/love_history_caused_by_willful_negligence/ (BL; ML is diagnosed with ASPD - antisocial personality disorder; stalking, attempted sexual assault on the MC by side characters, suicide, violence, non-con, dub-con, mild kidnapping and confinement)
https://www.mangago.zone/read-manga/happy_sugar_life/ (not for the faint of heart; features an extreme platonic or ambiguously romantic love between a high school girl and a little girl; grooming, kidnapping, confinement, pedophilia; NOT A GL!)

The beginning is hard to get through bc of how crazily obsessed the mc is but if you can get through the first 15 chapters it’s so worth it https://www.mangago.zone/read-manga/checkmate_1/

Brutal: Satsujin Kansatsukan no Kokuhaku
For your murder
Tomodachi Game
Olgami