In Theophany by Eric Perl, when covering Dionysius on the nature and causes of evil, Perl ends with a wonderful explanation of the fact that any apparently successful theodicy is itself evil. Here is the full passage:
For Dionysius, evil is privation and lack and weakness and asymmetry and failure [usually translated as “sin” but literally having the negative meaning “missing” or “failing”] and aimless and beautyless and lifeless and mindless and irrational and purposeless and unstable and causeless and indeterminate and unproductive and inactive and ineffective and unordered and unlike and limitless and dark and insubstantial and itself no being whatever in any way whatsoever.
Dionysius’ inability, or rather refusal, to assign a cause to evil, then, marks not the failure but the success of his treatment of the problem. To explain evil, to attribute a cause to it, would necessarily be to explain it away, to deny that evil is genuinely evil at all. For to explain something is to show how it is in some way good. “Tout comprendre, c’est tout pardonner.” Only by not explaining evil, by insisting rather on its radical causelessness, its unintelligibility, can we take evil seriously as evil. This is why most “theodicies” fail precisely insofar as they succeed. To the extent that they satisfactorily account for or make sense of evil, they tacitly or expressly deny that it is evil and show that it is in fact good. Dionysius’ treatment of evil, on the other hand, succeeds by failing, recognizing that the sheer negativity that is evil must be uncaused and hence inexplicable, for otherwise it would not be negativity and would not be evil.
It has been wisely remarked that any satisfactory account of evil must enable us to retain our outrage at it. Most theodicies fail this test, for in supposedly allowing us to understand evil they justify it and thus take away our outrage. For Dionysius, however, evil remains outrageous precisely because it is irrational, because there is no reason, no justiﬁcation for it. The privation theory of evil, expressed in a radical form by Dionysius, is not a shallow disregard or denial of the evident evils in the world. It means rather that, confronted with the evils in the world, we can only say that for no reason, and therefore outrageously, the world as we ﬁnd it does not perfectly love God, the Good, the sole end of all love. And since the Good is the principle of intelligibility and hence of being, to the extent that anything fails to partake of that principle it is deﬁcient in being. The recognition of evils in the world and in ourselves is the recognition that the world and ourselves, as we find them, are less than fully existent because we do not perfectly love God, the Good.
For a little more context, just before this passage in Theophany by Eric Perl, there is a fascinating summary of Plotinus defending an incoherent idea that matter is evil. Proclus rejected this as did Dionysius, both claiming that matter must be good. Here are the details regarding Proclus and Dionysius on this point:
Proclus differs from Plotinus by expressly rejecting the doctrine that evil is matter and that, as matter, it is necessary. He argues, more consistently than Plotinus, that “if matter is evil, one of two things is necessary: either to make the Good the cause of evil, or [to make] two principles of beings.” Either alternative is unacceptable. “Since matter is from the Principle, even this has its entrance into being from the Good. …Nor is evil from the Good.” To say, as Plotinus does, both that matter is evil and that it proceeds from the Good leads to absurdity: “Thus the Good will be evil, as the cause of evil, but evil will be good, as produced from the Good.” Proclus further argues that matter, precisely in that it is a necessary aspect of the sensible cosmos, cannot be evil: “But if matter is necessary for the All, and the cosmos would not be ‘this all-great and blessed god’ if matter were absent, how can the nature of evil still be referred to this? For evil is one thing, and the necessary another, and the latter is such that [the universe] could not be without it, but the former is privation of being.” By denying Plotinus’ identiﬁcation of evil with matter, Proclus thus avoids the difficulty of claiming that evil is a necessary condition for the good cosmos.
…[Dionysius] expressly follows Proclus in denying Plotinus’ “notorious” position that “evil is in matter, as they say, in that it is matter.” Dionysius argues, first, that “if [matter] is in no way whatsoever, it is neither good nor evil. But if it is somehow a being, and all beings are from the Good, this too would be from the Good.” He goes on to take up Proclus’ cogent argument that if matter is necessary, it cannot be evil: “If they say that matter is necessary for the completion of all the cosmos, how is matter evil? For evil is one thing, and the necessary another.” Whatever is necessary for the perfection of the whole is not evil but good. If, as Plotinus argues, matter is necessary, then it cannot be evil. This argument is effective not only against Plotinus’ doctrine that matter is both evil and a necessary consequence of the Good, without which the (good) cosmos could not be produced, but also against all attempts, such as have been made from antiquity to the present, to explain the evils that occur in the world as necessary contributions to the perfection of the whole. Any such theory, as Dionysius here points out, does not explain evil but rather explains it away by claiming, in effect, that it is not really evil at all.