Therefore (as a result of everything in chapter six), we can't really speak of an "efficient cause of an evil will"--such is "like trying to see darkness or hear silence." This is not to say we can't know anything about an evil will, since of course we can know something about darkness and silence, but it is to say that we don't really have a "something" to know--"only the privation."
Therefore, God Himself can never do evil because He is in His nature "can never and nowhere be deficient in anything, while things made out of nothing can be deficient." God is the fullness of Being, while we are deficient when we reject this fullness and embrace instead some lesser good, and so become deficient in our natures. This is something we do to ourselves, "that is why the punishment which follows is just, since the defection was not necessary but voluntary." Augustine here is relentless in his condemnation of our fallen wills:
Thus, [for example] greed is not a defect in the gold that is desired but in the man who loves it perversely by falling from justice which he ought to esteem as incomparably superior to gold; nor is lust a defect in bodies which are beautiful and pleasing: it is a sin in the soul of the one who loves corporal pleasures perversely, that is, by abandoning that temperance which joins us in spiritual and unblemishable union with realities far more beautiful and pleasing... In a word, anyone who loves perversely the good of any nature whatsoever and even, perhaps, acquires this good makes himself bad by gaining something good and sad by losing something better.How much, then, do we deserve hell when we sin through what is good?
Augustine here restates the fact that an evil will is one which has turned away from God. He now asks how the good, unfallen angels managed to remain unfallen while their fellow angels turned to evil. Did they do something within themselves that gains them extra merit for the generation of a good will? Of course not, because if they didn't have the good will to begin with at their creation, they had an evil will (which, we should remember, is the absence of a good will). We must, then, conclude that God gave them an extra benefit, namely in order for them not to fall God must "first awaken in the will a greater longing for this union [with God] and then fill the will with some of His very Being in order to make it better." All angels, in other word, were made equally good and equally desirous of union with God, but some angels were given, for lack of a better phrasing, extra grace that they might more greatly desire union with God, and had that desire met. Others had sufficient desire, but chose to turn it in a different, evil, direction. (And yes, this is yet another unsatisfactory explanation--and Edwards' lengthy recapitulation of the argument isn't much better.)
Now, however unsatisfying this answer may be, Augustine's pastoral point is a good and true one: both unfallen angels and redeemed man are dependent for their state on the grace of God alone. It is this outpouring of God's love into the hearts of men and angels that unites us into "a single community, one City of God, which is also His living sacrifice and His living temple." Which brings us from the unfallen angels to mankind, to which Augustine now will turn.