Disclaimer: I don't own the Dresden Files.
Course Correction
It's not often that the will of an archangel conflicts with the will of a Sword.
Ms. Murphy was everything Fidelacchius could have desired in a Knight, Uriel knew. Martial prowess and a battle-ready spirit combined with compassion and a strong sense of duty. There weren't many people in the world who would be as effective with Fidelacchius as Karrin.
Or who had the potential to cause as much damage. At the moment, Uriel had no lips to purse, but a massive tongue of flame whipped around for a few moments as it streamed out of the gigantic form of Uriel's true body. Ms. Murphy wondered at the Knights' mandate, and she wasn't willing to ignore her doubts. So far Karrin only questioned the Denarians right to leniency if they cast aside the coin, but what else would Karrin question in time? Would she question the very legitimacy of Heaven itself? That, Uriel could not allow.
Few realized it, but the Swords were not creations or implements of Divine Will. Heaven had assumed control of commanding the Knights, and at times Heaven could direct the Blades to appropriate wielders, but the Swords of the Cross gained their powers from the will of mortals. There was nothing to prevent the Knights from turning the Swords against the agents of Heaven, and the Blades were immensely powerful; they were easily capable of harming an archangel.
Uriel couldn't chance the possession of Fidelacchius to one who might reject Heavenly Authority. And therein lay the fundamental difference between what the Swords and Heaven wanted in the Knights. The Swords wanted heroes; Heaven wanted servitors.