Augustine is sometimes accused of ontologizing the effects of sin. City of God 14.13 seems to support this: “When [Adam] turned toward himself . . . his being became less complete than when he clung to Him Who exists supremely. Thus, to forsake God and to exist in oneself - that is, to be pleased with oneself - is not immediately to lose all being; but it is to come closer to nothingness.”
Further, the possibility of the fall is attributed to Adam’s creation from nothing: “only a nature created out of nothing could have been perverted by a defect. Thus, though the existence of the will as a nature is due to its creation by God, its falling away from its nature is due to its creation out of nothing.”
The Latin for the quotations above is: Sed vitio depravari nisi ex nihilo facta natura non posset. Ac per hoc ut natura sit, ex eo habet quod a Deo facta est; ut autem ab eo quod est deficiat, ex hoc quod de nihilo facta est. Nec sic defecit homo, ut omnino nihil esset, sed ut inclinatus ad se ipsum minus esset, quam erat, cum ei qui summe est inhaerebat. Relicto itaque Deo esse in semetipso, hoc est sibi placere, non iam nihil esse est, sed nihilo propinquare.
Maybe Augustine is ontologizing sin, making sin a metaphysical deprivation, a leakage of some stuff called being. But there’s an alternative account. His claim that only a creature made from thing could be perverted by a defect needs to be taken in context of his entire doctrine of creation. There are, for Augustine, only two sorts of beings: Those made from nothing, and God. God is and always has been what He essentially is; He is immutably all he is. His existence and attributes cannot be stained by anything. So, to say that only a creature made from nothing could be perverted by a defect is merely saying that only a creature can be perverted by a defect.
On the other point, Augustine might be doing precisely the opposite of what he’s charged with. That is, he is not turning sin into an ontological reality, but defining ontology in terms of relation to God. On this account, full “being” would mean being fully in a union of love with the God Who Is. Sin is a lurch toward non-being precisely because being means being-in-union-with-God. This account is suggested by the contrast between lesser and greater being. The two are opposed in terms of the inclination and direction of the person: Sin means turning to oneself, and this is a deprivation of being because it is means turning aside from God; Adam was in a state of “more complete” being prior to sin because at that time he was clinging to God (ei qui summe est inhaerebat).
In short, Augustine might well be struggling toward a relational ontology.