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I think the obsession angle is pretty good. Dark magic makes its users, or… recipients obsessive. Though I think Twilight is a good example of how even ordinary sparkly magic can make you obsessed with, say, stopping an impending disaster.
So here's my theory. Magic can create all sorts of things, from crystals to moustaches. Whatever is created or grown with magic isn't as good as the real deal though. Either it disappears or is lower quality or explodes or something. But that doesn't change the fact that magic is creating it. Dark magic is magic that creates magic.
I think dark magic is what you get when you try to make a spell self feeding. Using magic to cast magic, that sort of thing. Its effects are all messed up and twisted because firstly recursion is a difficult concept, and secondly because magic created magic isn't quite "right" compared to the original, just like anything created by magic. And the reason it is cheap and immensely powerful is because the unicorn only need summon enough magic to start it, not needing any to keep it building.
That explains why it gets out of control, and why they don't want anyone using it. It's basically at risk of fork bombing the universe. Ponies get obsessed over it because the more you use the more you get back, a positive reinforcement loop. In a way they become part of the spell ever increasing in severity. Also explains why it's a mess to clean up, because anything created with dark magic is covered in the stuff, and trying to remove it could set off another cascade reaction.