A manga where main character is a hero, or good willed good natured guy but something happens that makes him ruthless. Here are a few examples;
Cavalier of the Abyss http://www.mangaupdates.com/series.html?id=36319
Übel Blatt http://www.mangaupdates.com/series.html?id=2291 (best example)
Mahou Tsukai Kurohime http://www.mangaupdates.com/series.html?id=464
gah I can't think of anymore I swore I thought of lots when I was laying in bed

I would appreciate any help! Shounen, Seinen, Jose, Shoujo any genre
Even if you don't think it fits exactly, recommend it regardless I will take a look! Appreciate it~
P.S.
I was wondering if anyone knew of a story of main that turns into a villain not anti hero, a villain
(besides death note)

Edited
Mangas I've read that have been recommended
The Breaker
Legend of Maian
Berserk
Code Geass
Zetman
Maouyuusha
Mirai Nikki
Kiseijuu
Fire Candy
Kimi no Knife

The Breaker

His master goes super sayajin and becomes a villain in the second season

The legend of maian
Necrophile of Darkside Sister

Thanks for reminding me about it anything else come to mind? appreciate the help

Oh, forgot one of the oldest Anti-hero manga Berserk 😀

I haven't heard or read this "Necrophile of Darkside Sister" looks interesting thanks!

people rarely reply to my threads are they too difficult or do they just not like me =( so I appreciate it keep them coming.
I usually come here as a last resort after searching and asking a buddy of mine whos into manga as much as me

you can try code geass anime its similar to death note

You mean Kurohime...

Tsukihime is nothing like Kurohime.

gss93 @ Yeah thanks for reminding me any others you can think of?

pthb @ Lol sorry don't know why I wrote tsukihime xD

a fairy tale of the demon lord, good guy becomes bad guy( hes still the main char tho lol)
Maoyuu Maou Yuusha, hero becomes maou partner ( they are in middle ground here and there isnt gore)
ED. oh almost forgot ZETMAN, and of course devilman.
Captain harlock may count but...

Thank you they the last two you mentioned look interesting, I had already started demon lord but never really gave it a chance you perhaps gave me a push to read it

Yo man! You should have put more mangas that you know of I suppose, lazy man, or perhaps, lazy noman 😔

Kami no kodomo - He becomes evil because he's born and didn't want to be born. It's totally justified 😔 anyone would react like him. It's hillarious~

First chapter of Zenryou naru Itan no Machi with the father who has grown tired of his family's constant bullshit 😔 anyone would react like him. It's hillarious~
Just read it now. It's just a few pages long.

I'd say Code Geass too, but you don't seem to be into that. Possibly one of the least disappointing endings I've witnessed 😔 Anyone would react like him. It's hillarious~

Hahahaha Naruto, and you can just skip everything that doesn't revolve the Uchiha clan. It's not hillarious. I don't think anyone would react like him... Not at all.. and it's not even funny~ 😔

Vinland Saga - It'sss.. his father dies by the hands of vikings, so he has no choice but to join the vikings that killed him just to have his revenge. He becomes quite the bitch actually - if you like bitches - this is it. But then, he realizes that and regrets his cruelty. It's hillarious and interesting. Anyone would react like him~ 😔

It doesn't really fit but these are in some way "evil", but not like you want them too I suppose:
Liar game - he kind of becomes evil because of his mother being conned, though he really isn't evil, yet. Still think he's hiding something though. Interesting read~ 😔
One Outs might be alright if you're into that. Don't know if I'd call him evil, but he's quite full of himself in a way that can be perceived as evil. Interesting read~ 😔
Kiseijuu - maybe.. people get Parasited, (yes, it's a verb when capitalized), and so their parasites have the need to Parasite on humans. The main character just grows cold-hearted, he's not evil I suppose. Interesting read~ 😔
A Fairytale for the Demon Lord - perhaps~
Shingeki no kyojin - could be.. he becomes a giant which pretty much is the threat of that world~
Dorohedoro - maybe~
Himizu - absolutely definetely maybe. But it's not that good~

EDIT: just search for revenge, or other typical stuff that would make someone evil. Not sure how these search engines here work, but I like this site so that's why I use this.

phantom requiem for the phantom anime
maybe black lagoon

I also like these type of storylines. It's a shame they're so rare.

Anyway, I recommmend.

Anne Freaks
Mirai Nikki

Narutaru - you gotta read till the end

she kill EVERYONE

Ares - one of the friend become the big antagonist. i dont think he's good natured from the start, but you kinda like him so...

Eden: It's an Endless World! - he's just a kid but then he become

a hitman then a drug addict

Kimi no Knife - just a regular school teacher become a hitman.

7 Seeds - okay , this is a big stretch, but i think the guys from summer a is kinda like this.

Battle Royale - the 'hero' is still a big pussy till the end, but i dont consider him as the main character and i consider everyone is the hero of their own story. well, if everyone is out for themselves, normal kids can become crazy.

Fire Candy - this is a very big stretch, i think the protagonist is crazy from the start, but you might want to see his descent to the abyss.

Koroshiya 1 - this is also not really what you want. but i think ichi is basically a mentally challenge person, one cannot really blame him for doing what he did.

Thanks for all the recommendations I will look at them when I have time. I am at the brink of collapsing, due to family issues ^_^ thanks again I have updated the mangas recommended that I have read if you recommended one that I read, please don't feel it wasn't taken into account I appreciate you reminding me of it. So that I can give more examples