Are there any romance mangas with an unstable male love interest? Possibly, he has large problems in life or mental issues? Smut/mature themes are totally allowed (romance should be prevalent)! No yaoi or yuri, please.

Example(?):
Cheese in the Trap. The male love interest is a possible sociopath.

(Sorry! Don't know how to link mangas!!)

Yeah, my boyfriend's pretty cool. But, he's not as cool as me.

I would recommend:
Bokura no Koi wa Shi ni Itaru Yamai no You de
(mentally unstable male and female lead, Smut)
Devils Line
(vampire - human relationship, Mature)

Maybe also:
Haru no Houtai Shoujo
(protagonist has supernatural powers and hurt the female lead)

Maybe Hakuji, Savage Garden, American Ghost Jack, Flowers of Evil (this one is a bit too sad for me 🙁 ), Korosareru nara, Isso Sakura no Ki no Shita de
Hope I was able to help somehow 😀

Kare Kano
Orange (TAKANO ichigo)

Ooh! Thank you so so much for the recommendations! They seem super interesting, I am gonna read them right now 🙂

@okonomiyaki-san
You definitely helped me, these recommendations are really good. Thank you so much!!!

Yeah, my boyfriend's pretty cool. But, he's not as cool as me.