Taking Advantage.

More than pride, she didn't want to join the fight against the Shadow of despair now because she wanted her fellow Supreme beasts and their alliance to suffer. They have been pressuring her and limiting her progress with their alliance so she wants them gone. 

She is very dangerous so they wanted to get rid of her. It is only because the cost will be too much that they didn't attack her outright. At least not until they eliminate the Shadow of Despair. But they threatened her in the meantime. She had to keep her disease in check. 

She could have disobeyed the alliance and killed more people in the plane but she didn't want to fight them because she is very easy to kill despite her ability to cause the extinction of all life throughout the plane.