The devil was not created sinful, but rather was created good (as were all things) and then fell of his own accord.
We see in creation an order of values that is arranged according to rational principles. For example, we value sentient beings over non-sentient beings, and living things over non-living things, yet we would all rather have (according to Augustine's example) food (non-living) in our house than we would mice (living); yet sometimes we would choose mice (non-sentient) over the company of bad men (sentient).
And so, we can value the Redeemed of God over the fallen angels without violating the rational structure held up by Augustine.
God made everything good, such that even when a part of it sins His purposes are still worked. So God retains His Sovereignty even over the rebellious:
But God, as He is the supremely good Creator of good natures, so is He of evil wills the most just Ruler; so that, while they make an ill use of good natures, He makes a good use even of evil wills.Chapter 18:
This contrast with evil if nothing else highlights the Beauty of the Highest Good. When there is rebellion against God, it remains under His will and demonstrates how truly Worthy He is.