GLs with a black-flag love interest:
https://www.mangago.me/read-manga/kill_me_now/
https://www.mangago.me/read-manga/tatoe_sekai_ga_owattemo_watashi_wa_kanojo_wo_erabanai/
https://www.mangago.me/read-manga/kitanai_kimi_ga_ichiban_kawaii/ (toxic relationship, features some extreme fetishes such as age regression/babyplay, vomit kink, period blood kink, etc.)
https://www.mangago.me/read-manga/i_m_more_dangerous_than_you/ (age gap, teacher-student relationship but both leads are 18+, bullying and harassment, kidnapping and confinement, domestic violence)
https://www.mangago.me/read-manga/manipulate_my_heart/ (age gap, teacher-student relationship but both leads are 18+, bullying and harassment, kidnapping and confinement, homophobia, human experimentation)
Toxic GLs:
https://www.mangago.me/read-manga/gunjou/
https://www.mangago.me/read-manga/hone_no_nazuki_made_aisu/
https://www.mangago.me/read-manga/seasons_in_bloom/
https://www.mangago.me/read-manga/the_chain_of_youth/
https://www.mangago.me/read-manga/your_fetters/
https://www.mangago.me/read-manga/zenbu_kowashite_jigoku_de_aishite/
https://www.mangago.me/read-manga/savior/
https://www.mangago.me/read-manga/my_brother_and_i_are_rivals/
https://www.mangago.me/read-manga/on_a_leash/ (SPIN-OFF: https://www.mangago.me/read-manga/let_loose/)
https://www.mangago.me/read-manga/black_white_sal_jiang/
https://www.mangago.me/read-manga/officer_you_can_no_longer_hide_your_tail/
https://www.mangago.me/read-manga/indulging_a_crazy_beauty/
https://www.mangago.me/read-manga/i_love_amy/
https://www.mangago.me/read-manga/the_barefoot_nina/
https://www.mangago.me/read-manga/our_love_is_disgusting/
https://www.mangago.me/read-manga/ubai_ai/
https://www.mangago.me/read-manga/shouwaru_tensai_osananajimi_to_no_shoubu_ni_makete_hatsutaiken_o_zenbu_ubawareru_hanashi/
https://www.mangago.me/read-manga/serenade_keum_kyesoo/ (toxic relationship between FL and prominent side character; age gap, teacher-student relationship)
https://www.mangago.me/read-manga/warikitta_kankei_desukara/ (age gap, teacher-student relationship)
https://www.mangago.me/read-manga/chirizokonai_no_hiraeth/ (suicide and suicidal ideation)
https://www.mangago.me/read-manga/naraku_no_hanazono/ (incest and child sexual abuse [not between MC and FL], abortion, mild violence)
https://www.mangago.me/read-manga/love_thy_neighbor_2/ (age gap, adultery, religious themes, attempted kidnapping/human trafficking)
https://www.mangago.me/read-manga/the_nest_of_eternity/ (age gap, pseudo-incest [kind of, they're not biologically related iirc], non-con)
https://www.mangago.me/read-manga/immoral_parody/ (age gap, non/dub-con)
https://www.mangago.me/read-manga/outro/ (adultery, abusive parent)
https://www.mangago.me/read-manga/yodaka_meguru/ (adultery)
https://www.mangago.me/read-manga/my_girlfriend_s_not_here_today/ (adultery)
https://www.mangago.me/read-manga/efude_to_kurokinu/ (one-shot; adultery)
HOLY SHIT THANK YOU SO MUCH （/TДT)/
give me toxic fucked up yuri. like the yuri equivalent of all those black flag bls