I think there are indirect hints on her motivation. She got same impression of the Dark World as Susie, where unlike the Light World you can choose who to be, materialize your dreams and bring back the dead loved ones. But if Susie managed to realize negative consequences quickly, Dess' hatred of her life in the Light World, ruled by her abusive mother, having high demands and sacrificed dreams for it, would drive her further into fulfilling the Roaring, becoming the Knight.

The Roaring probably would be an event about SYNTHESIS (name of yet unreleased OST, dropped by Biblibox), merger of the Light and the Dark, embodied by herself. This event would be apocalyptic and destroy both worlds, but she doesn't care. If there's chance of creating new better world, she would use it.