I'm looking for a semi serious manga where the main character or prominent side character joins the dark side after either being forced, brainwashed, threatened, or pretty much decided that the dark side is just better.

I guess a shoujo manga where the girl falls in love with a dark evil guy is ok, just looking for a bit more serious.
Extra points for super powered evil side.
I would also like fantasy but non fantasy is ok.

Another type of manga I'm looking for is when the good main character uses an "evil" power to fight for not specifically good, but their own agenda, kinda like Kurozakuro.

Thanks

Ratman. not much of it has been quality scanlated, though. That's supposed to change at some point though >.>

You could also try Death Note or try watching code geass.

EDIT: mmk, i'll actually go down my lists
Destroy and Revolution
Shamo - this was actually too dark for me >.>
Ubel Blatt - matter of opinion. I think he was in the wrong for trying to kill EVERYone
Red Eyes - similar plot to above, only he's more clearly on the good side.
Akumetsu - similar to destroy and revolution as it's about a terrorist. only this guy is depicted as being more justified.
Ares the last arc is about a major betrayal...kinda.

this might not be what you're looking for
Zippy Ziggy - a comedy about a guy who never palys fair because there's no point. 😛
Kyou kara Ore wa!! - another comedy about two guys who change schools and become delinquents. they're still kinda decent people though...well one of them was never decent but it's hilarious <.<
Kyou kara Hitman ... probably isn't what you're looking for as it's episodic
Embalming - The Another Tale of Frankenstein - kind of... the main character agrees to destroy all frankensteins, even though th eorganisation isn't shown to be that bad, so far..

I remember there was a thread similar to this last year. someone mentioned one where the main male character turns evil at some point...i don't remember what it was. May have been this Mahou Tsukai Kurohime. I havne't actually read it though

Maybe..

Tower of God

           and  

Phantom Wizard

death note sounds right up your alley

I'm gonna say my Fav manga!
Superior - the girl was forced to become the Demon King and she uses her power however she wants. Also she laughs like crazy while killing. it's a fantasy story.
Also Mahou Tsukai Kurohime - she can kill without even winking and she is the stronger witch on the whole world... and everybody hate her >.< (also a fantasy story)

Find me on DeviantArt w
http://xkocicax.deviantart.com/

Berserk Kinda fits, Guts thoroughly enjoys killing random people, and Grifith is just plain fly.