I'm looking for any manga/anime that has this type of scenario:

Basically, a character (Originally Good) accepts a deal with an Evil/on the Opposite side/Morally Grey Character. A "we can rule together" kind of situation, where the "Good" character accepts the deal. And, it has to be a main plot point, not just like some side character/back-story (unless the entire manga/anime is about the back-story).

Some requirements:

• Preferably Shounen/Seinen/Action.
• The main character is the one that accepts the deal.

What I don't Want:
-Romance
-Yuri.

What I'm fine with:
-Yaoi/BL/Shounen-ai

What I've read that I think can fit these type of situations:
Maoyuu Maou Yuusha

Thanks in advance.

Priest : The first that comes to mind and it fits perfectly.
Dendrobates : Maybe... It's part of the past of the MC which is revealed relatively late in the story.
Jormungand : That's how the MC gets recruited to be a mercenary. Later on he gets similar deals ("we can rule the world together.. blah blah") from the female lead.
DICE: The Cube that Changes Everything Kinda. The male lead seems to still avoid the fate of turning evil, but most characters in this story don't. Some romance in this one.

And here is a manga where it is subverted : Ratman. Goody two shoes wannabe superhero gets recruited by evil organization, and becomes a super villain. It's played for laughs though.

Rebirth it's like that if you include the flashback,
good characters turned evil, both of them, and in the main-storyline the main represents the darkness (vampires) fighting against the good side (angels and gods)
no idea about him trying to master light magic, it wasn't completed.

its hard to find bad mc...

Alive: Saishuu Shinkateki Shounen not mc but his friend
Eden - It's an Endless World! the transformation seems gradual but try it

another random post of mine >.< ...(hoping its useful in some ways) :

berserk
ubel blatt
hellsing
A Fairytale for the Demon Lord
Crimezone
Vinland Saga
Kimera
Mahou Shoujo Site
Variante

I haven't caught up, but I heard that Tokyo Ghoul is heading in this direction.