I think the Black Mage could be good

White Mage didn't corrupt out of nowhere. His origin story took place over several decades as he slowly became more and more blinded by his own ambition of discovering the ultimate light. He initially wanted it to do good with, but after years and years of pursuing it, it got to the point where he was more obsessed with finding it than actually using it for the good of the people. Finally, he snapped when he came to the realization that after decades of research and hard work, it turned out there was an upper limit to the power of light, but darkness' power was theoretically infinite. Since he was more obsessed with finding the theoretical limit of power at this point than actually using the magic for good, he decided to shift gears and start researching the darkness instead.

It's just the classic story of a hero losing sight of his ideals and becoming the villain.