The mc must be a male. And the concept has to be him trying to be good or avoid being a monster. I already know Tokyo ghoul, parasite, blue exorcist. So nothing popular that is already known. Something similar to kaneki's character from tokyo ghoul when he rejects his ghoul side.

Dawn - Tsumetai Te *This fits your description exactly
Rosario to Vampire - Season II
Rosario to Vampire *plot becomes a lot better in part 2
Berserk *One of my faves, it takes time though.
Nurarihyon no Mago Does this count?
Magane no Agartha
World Embryo *Very good, what you'd expect from the mangaka of Chrno Crusade !
Tales of Innocence *some elements
Kurozakuro *fits your description exactly
Ajin *some elements
Übel Blatt *some elements
Devilman *Old classic
Priest *very good!
Alive: Saishuu Shinkateki Shounen *some elements, MC doesn't really turn into a monster perse, but the plot has to do with viruses, one of my faves!
Hito Kui *Some elements, not a lot of chapters out at the moment though. Big twist at the end.

There are others, but these are the only ones I could think of at the moment. You can always lurk around the forum more, who knows? You might be able to find others with tastes which aligns with yours.

Like this thread: http://https://www.mangaupdates.com/showtopic.php?tid=47216& hl=male%20transforms%20into%20monster

You can also look into certain categories to refine your search
ie: Human Becomes Demon/Monster

Last edited by Miaxchan at 10:19 pm, Feb 9 2015

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Kirihito and parts of Hi no Tori too. Dorohedoro is about that 100%. Claymore has such a theme. No "demeons" can befound here but your theme is there:
Jisatsutou/Destroy and Revolution/The World Is Mine.

Last edited by residentgrigo at 10:50 pm, Feb 9 2015

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