Well, there's a good question there.
I actually think that assuming that Asmodians are Demons, it's a abit odd. Since it's gonna be one of the two playable characters, it will be funny indeed seeing a Demon called "FluffyBunny" or wtv you can think off :P.
And no i don't think you have to act as a Demon, problably in your story mode, your path will be darker, while the Elyos path will be more angelical. But in the end, is the player decision that makes it look demonic or angelical
.. . unless Aion would have Hardcore RP servers. now that would make some sense according to your quoted text.