Like the title say, I'm looking for male demographic with female lead manga. But not just any manga. I'm want the antagonist of the manga to fall in love with the female lead or want her for specific reason or obsess over her. Well, I dont't care if the female lead can fight or not as long it male demographic and the antagonist just head over heels for her. If she can fight or anything, it just a bonus for me.

Thank you for your time for reading this post. I really thirst for some action or maybe something gore. Also that just a bonus for me.

¤ I'm a female human that want to explore the universe so I can find another Earth. ¤
♢ I'm a female human that want to sleep underwater cuz I find it surprisingly calm ♢
% I'm a female human that want to become a scientist but I can't because I don't like math, add-math and physics. %
@ I'm also a female human that just to stay in my room and wasting my time with favourite anime and manga. @

I'm also a female human that obsess with reverse harem related.

Hi, there!

That description sounds like Fate/Stay Night. There you have an antagonist called Gilgamesh who is obsessed with the female co-lead (Saber/Arturia). However, it's not exactly what you're looking for; and also, that story is better in the anime version because you have more of those moments of Gilgamesh's obsessive love (or just obsession) for Saber in it.

Another action manga (one of my favourites all time) that involves a cool female lead is Claymore. Nevertheless, in this story you won't find any kind of romance between the MC and the antagonist. It's a great read, anyway.

Fukushuu Kyoushitsu it's somewhat close to your description too (it's a horror/psychological manga, though). There the female lead is taking revenge on all the people who humiliated her, and there is a slight touch of romance with a guy who is thought to be the 'bad one'.

What about Kakegurui? This manga has a smart and cool female lead. However, the 'romance' is more like a 'sismance' (even if it's hinted that MC's male friend is insterested in her the manga is written in such a way that Shoujo-Ai is stronger).

Those are the titles I have read and that I can mention in the demographic you're insterested on.
Aside of them, I can suggest you to take a look to Josei genre, in it you'll find many romances like the one you want to read.

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Chang Ge Xing

Basara

Azumi
Various male antagonists/rivals fall for a powerful female samurai/ninja/assassin. There isn't a single love interest though (yet?)

Ran to Haiiro no Sekai
If I remember correctly, the main antagonist is obsessed with the female lead. There's a huge age gap though.

Zettai Karen Children
I haven't read this personally, but there are similar themes with the previous series I mentioned, however the antagonist appears less.

Kimi Shinitamou Koto Nakare
Not exactly what you requested, but I thought I'd throw this one in since there's ton of action and the male lead is obsessed with the female lead, even going as far as to kill girls who insulted her. He's pretty much an anti-hero.