HOLY SHIT WHY DIDN'T I SEE THIS SOONER?

Okay, so I was going reading through 8-bit theater (again), and I get to this.

We know that Twilight can travel to other worlds because of the Crystal Mirror.
We know that Twilight, being the curious mare that she is, would likely make multiple trips through the portal.
We know that things often go wrong for Twilight.

So, at some point, Twilight makes a trip through the Mirror, and winds up in the world of FF1. The first person she meets is Fighter, who, being Fighter, drives her mad while he's trying to befriend him. When it's time for her to return through the Mirror, Fighter screws things up so badly, it totally destroys the portal. Twilight can never go back to Equestria, and the person responsible for it is too stupid to realize what he just did, and is determined to stay by her side. This causes Twilight to completely go over the deep end, creating the alternate personality of Black Mage.

The fancy, girly rooms when he had control over the Temple of Fiends? That's Twilight trying desperately trying to remember her home.

The hatred for Fighter, despite him only ever being friendly to him? How would you like being stuck with the person that destroyed your only chance of getting home.

Wanting to destroy everything? Twilight just wants to see her friends again, and is convinced that the only way to do so is to destroy the world.