New Maplestory Elements?

Actually, there's a storyline reason for that. Everyone that tries to do dark magic becomes evil, creates an evil something, or something to that effect. Faust was a result of Grendel screwing with dark magic.

The ordinary Zombie Lupin are Grendel's fault. Faust is caused by Reef/Leef, who was sort of snubbed by Arwen and set a huge curse.

But yeah, in general this is true. Shammos turned into an ugly demon thing for messing with dark magic. The Hobi in the minar forest are hinted to be related to this as well (I'm hoping Hobi's Monster Book entry will clarify this). Grendel's foray into dark magic led to the Zombie Lupins. Those are the best examples, there are a few others (I want to say Book of Ancient is dark magic, too, but I'm not certain)

Still doesn't mean you can't be a Dark-magic user, imo. They could make a seperate storyline or class system for that, albeit, slightly different than the other mage classes. :/

On the other hand, I get the feeling that if they make Black Magician loyal classes (The opposites of Cygnus, as was alluded to in the Korean developer interview) that the mage will, naturally, use dark magic. Also don't forget that the Nightwalker's sprite is dark-powered. I'm guessing this allows Nightwalkers to use dark attributes without being corrupted themselves since the power is actually the sprite's. So yeah, in some ways they already have done this.