I'm looking for manga with a hero that fights against evil using the power of evil.

Ao no exorcist is a pretty perfect example.

this theme is also appears in:

naruto
bleach
kurozakuro
dragon eye
claymore
aflame inferno

However, in bleach and naruto the characters with "inner evil" also have power in them derived from good(or at the very least power that doesn't come from evil).

I'm looking for something where the power is mostly based on evil, and threatens to consume the hero (more like claymore).

Feed your head.

666 Satan~

Witchblade - isn't that good though.
Kiseijuu - maybe.

Hmmm...maybe Guyver?

Being my favorite type of shounen, I wish I had more to recommend and some that applied more to the theme. >.>

But the two that can sorta relate that I picked right of my list are:

World Embryo (sorta kinda)
Dawn - Tsumetai Te (it's more like the more he's using it, the more he's becoming what he doesn't want to be... shrug)

[color=purple]Why, hello there!!![/color]

ARMS
Chaosic Rune
Priest
AR∀GO

And there would be Zetman, Devilman but they fit your description only to a certain degree.

A bit of a stretch: Shikabane Hime.
Also if you don't mind excessive gore, try Hellsing.

Devilman
Berserk
Tokko
Dead End powers aren't evil but they do consume him.

[img]http://i53.tinypic.com/nq73ao.jpg[/img]

Quote from purple716

Is there any manga where the male lead has an illness known as stomach ulcers.Yaoi are fine top.

togari

Sakurahime kaden if you don't mind shoujo.

If you've ever thought or said "Nice Guys finish last" and really meant it, then you should probably read this LJ post by DivaLion. It's incredibly insightful whether you're male or female.

From a bumper sticker I like:
"If you're gonna ride my ass, at least pull my hair."

Kamikaze Kaitou Jeanne, but it's shoujo.

Beelzebub maybe? Note that the power doesn't risk consuming him until chapter 20 or so.

i suppose...

pandora hearts
d.grey man (?)
majin tantei nougami neuro
chrno crusade (sorta)