I want to read an manga something like Lord of the Flies. Or something that is tragic like Romeo & Julliet but please no shoujos.

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Imagination rules the world. - Napoleon Bonaparte

All men are born with a nose and ten fingers, but no one was born with a knowledge of God. - Voltaire

Battle royale 😀 ?

Well, here are some that might fit your description:
Battle Royale
Alive
Bokurano
Hitsuji no Uta

The most "Lord of the Flies" feeling I ever get was from the anime "Mugen no Ryvius". There is also the manga but I can't guarantee for it (this Mugen no Ryvius). I think it could be bad only because it's been made after the anime and it's only 2 volumes (against 26 episodes).
There isn't anything else so similar imho to "Lord of the Flies" (except maybe Battle Royale, but I'm not so into it); something a little similar could be Mirai Nikki or the series "Higurashi no Naku Koro ni" (I'm sorry if I stated too much anime).

Tragic non shoujo love story:
Chrno Crusade, again for that part maybe anime > manga, but also the manga is good.
ES maybe
Kurozuka a little gore tough.
Shin Angyo Onshi but most of it is a flashback
Vampire Juujikai

Maybe they didn't match so exactly the Romeo&Julliet story, but they are all good for me.

What I like in Trivial Pursuit style. Pick your category:[img]http://i379.photobucket.com/albums/oo233/Reid4891/Manga/trivia-1.jpg[/img]

Drifting Classroom
Metro Survive

Definitely Battle Royale. It is effectively Lord of the Flies/Last Man Standing.

Basilisk; perfect with the Romeo & Juliet plotline, only here it's between two rivalling ninja clans that possess 'X-Men'-like powers. o__o
Berserk; can't survive more than he did...
Claymore; kind of. No real romance though.
Deadman Wonderland
Dolls
Full Metal Alchemist; survival manga in its own right with subtle romance sprinkled in between.
Immortal Regis
Priest
Toshokan Sensou: Spitfire
Zetman; great manga with lots of tragedy in it.

source: animenewsnetwork

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Wow. No one's mentioned Saikano yet?

[img]http://i27.photobucket.com/albums/c193/cyberfalcon1360/yukixharuhiml1.jpg[/img]
"Oh when you look at me like that my darling, what did you expect?"
Arctic Monkeys - 505

Basilisk - the setting is almost exactly like Romeo and Juliet cross with Naruto

Chrno Crusade - not really Romeo and Juliet kind of tragic, but you sort of know from the start there won't be a happily-ever-after ending for the main characters.

Nana's pretty tragic right about now

[img]http://img40.imageshack.us/img40/4158/1205047355313.gif[/img]

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Battle royale 😀 ?

Yeah already read battle royale the novel and I read Kouga Ninja Scrolls already also.

Protect Independent Film
All that we see or seem Is but a dream within a dream. - Edgar Allen Poe

Imagination rules the world. - Napoleon Bonaparte

All men are born with a nose and ten fingers, but no one was born with a knowledge of God. - Voltaire

Highschool of the Dead is a survival manga, if that's what you're looking for.
It's all about trying not to die! 🤣