Why not use the classic "kill everyone who is different."
Define "different". A mage is certainly different from a mundane. A Tevinter is certainly different from an Orlesian or Fereldan.
Alexius was not disillusioned, he was desperate. So was Erimond. Compare both to the noble and kind scholar Erasthenes, whose visage graces my icon. He was the foremost authority on the Old Gods, pre-eminent, and the Venatori punished him in their jealousy. How much worse would it have been if more graspers were allowed to glimpse power denied them by their inferior birth?
On the contrary, his imprisonment was inflicted by those who claim to be borne of Dreamers. His greatest regret was the prospective bondage of his former servant, who bore great power and love for the Imperium, and yet was previously denied proper eminence by her so-called "low birth".
We might as well be southerners, letting descendants of barbarians rule soi-disant (excuse the Orlesian) royal lines such as the Theirins and Valmonts.
Indeed, we might as well be southerners, who find authority and virtue in the ink of pedigrees and epitaphs, rather than the power wielded by one's own hand. It is a tradition that stinks of the bitterness and jealousy that infects the hearts of the accursed; those forced to walk the earth bereft of magic's gift.