Once Belius fell into Thrud's hands, the Empire would have to deal with the Divine Beasts on a daily basis. The Mad Queen had made no mystery of her intention to restore the Kingdom's borders to the position they had been after Valeron.
In other words, she meant to invade the Empire and once she got past the natural barrier of the mountain range, nothing would stand in the way of her troops. The Empress was trying to recruit as many Faes and Beasts as possible, offering equal rights for their kind, but very few had answered her call.
Thrud's deal was better since she also offered them Awakening and full disclosure of the secret of the violet core.
The only reason they had yet to join her army and the Mad Queen still lacked the numbers to start another campaign against the Gorgon Empire was that Awakened creatures didn't trust her.