In respect to moral arguments, solutions to the problem of evil and the testimony of scripture, we are usually left with the often conclusion to understanding God’s moral character as “perfect”, or “wholly good”. However, do we take God’s character as such to be a matter of faith? Isn’t it just as likely that God who is “wholly good” that he instead be “wholly evil”? We are reminded by the passage found in Descartes’ First Meditation:
Accordingly, I will suppose not a only a supremely good God, the source of truth, but rather an evil genius, supremely powerful and clever, who has directed his entire effort at deceiving me. I will regard the heavens, the air, the earth, colors, shapes, sounds, and all external things as nothing but the bedeviling hoaxes of my dreams, with which he lays snares for my credulity. 
How are we justified in suggesting that an “evil genius” of this sort found in Descartes’ meditation be one that doesn’t exist, but rather a perfect and wholly good God? First, let’s gather a proper understanding of evil.
An Augustinian View of Evil
Augustine in his famous spiritual autobiography had once asked, “Where is evil then, and whence, and how crept it in hither? What is its root, and what its seed? Or hath it no being?”  In a proper response to this problem, Augustine’s argument on evil can be schematized as such:
1) All things that God created are good.
2) Evil is not good.
3) Therefore, evil was not created by God.
Another argument under the same scheme could function as the following:
- 1a) God created every thing.
- 2a) God did not create evil.
- 3a) Therefore, evil is not a thing.
Augustine’s particular position was that “Evil has no positive nature; but the loss of good has received the name ‘evil.'”  In other words, evil by definition is the “absence of good”, and therefore has no affirmative nature attributed to it. This has a few interesting implications in respect to our question: Is God an evil demon?
Response to the Evil Demon
A few problems with the evil demon argument is namely that it is meaningless. First, if evil is the “absence of good” (which is a negative nature), and God is the ontological foundation of morality, then how can God exist if he too would have a negative nature? For:
1′) Evil is not a thing.
- 2′) God is evil.
- 3′) God is not a thing.
You can restate 1′) however you’d like: 1′) Evil has a negative nature; 1′) Evil is not a substance; 1′) Evil does not exist; and the conclusion will still substitute to a similar kin: 3′) God does not exist. Therefore, an evil demon by definition could not exist. Another response to the argument would be to state that since evil can be understood more or less as “unnecessary promotion of harm” and good as “contributing to the benefit”, then all moral acts/decisions would essentially be circular and good moral acts could not exist.
Since, if God were evil, there could be no good acts because evil acts would still bring harm and good acts would also promote harm since God being evil desired evil so doing good would bring about his desire and therefore be evil. The evil demon argument is clearly absurd!
-  Renee Descartes, Meditations on First Philosophy (Hackett Publishing: 2000) p. 107
-  St. Augustine, Confessions (VII: [V] 7)
-  Ibid.