You also have the dumbass beast prince who got too comfortable in his kingdom far away from the demon conflict that he believes they could deal with demons after betraying the empire.
Forgot to mention they want to do it on their own, as far as we know they don't have any dealings with demons. Which is dumber cause if they weaken the one dealing with demons they're next and they definitely don't have the numbers.
We already knew that the crown prince wanted to backstab the coalition as revenge for the slavery and discrimination of beast kin. I can't remember the king's position but he may have been convinced.