If my guess is right, Meredith had no such intention towards Rood, it's only the tower magician especially Mikel that harbour such notion.
Something must've happen in the demon community in long past, some sort of mutiny or coup that led Dk to be splitted away from himself, and followed by the demon waging war against human.
Dk bro must've willingly plan to let himself being used up to empower the human during that dire time. Actually he really do willing to help, but the consequence of him being chained up maybe is part of his willing plan, or it's an act of betrayal by human.