@Monkey D. Goku he literally said what they're doing is for the empire and the Prince is the key to have both authority and power to take over the empire. And maybe they used the Prince to eliminate all magicless people since that guy and the people behind him despised them. As we can see now, the Prince seems not hate people with no magic or commoners so bringing the Prince to their side is crucial to put their plan in place without intervention regardless of being brainwashed or the Prince free will.
I don't care about your taste!