Dr. Jang's evil genius rivals even Gelimer's. I mean, empty-handed 1000 years before she was born, she's able to manufacture endless animal-control devices and special mutagenic substances? And yet, she'd rather go looking for a long-gone demon of unknown powers, rather than ally herself with the Black Mage directly?
Spoiler
Because she's not inherently evil. The initial implication was that she was searching for the Antellion because its power could be used to fight the Black Mage. Once things went south for her, she stopped caring about that and just wanted to prove she was right. Even after that, she still made Narcian promise to keep the Antellion in Versial, because she didn't want it to fall into the wrong hands in the maple world.
Narcian is presenting himself as the victim in the conflict, so she sees Subami as the villain (she's also obviously got the hots for Narcian, which is factoring into this). The lengths she's going through to help him are more than a bit morally bankrupt, but she's not someone who would ally with the Black Mage.