In On the Fall of the Devil, St Anselm discusses the proper and improper uses of the concepts of willing and ability. And, as my recent post about justice mentions, Anselm also discusses the concept of evil/injustice as non-being. Properly speaking, a book cannot be written by Anselm, for a book has no will and does nothing. However, properly speaking, Anselm can write a book, have both will and ability to write. This is what we mean about using words properly in such contexts — what is their more precise denotation, rather than how we commonly use words to communicate and to woo women.
Anselm is not wooing women.
Taking as foundational for St Anselm’s discussions of God, we must have the rigorous intellectual and meditative work of the Monologion and Proslogion. God is not only that than which a greater cannot be thought. God is also the best, the perfect, the most just, the eternal. God is not merely just the way we may say that a court’s verdict is just. God is justice Himself. God is wisdom Himself.
God is being itself.
God, in his perfection, tends always towards goodness, truth, beauty, being.
If He did not, He would not be God.
This is a major component of the classic definition of God.
In On the Fall of the Devil, Anselm writes:
This [the discussion of speaking properly and improperly] is the origin our saying that God cannot do anything that is contradictory or perverse because God is so powerful in justice and beatitude, indeed, since beatitude and justice do not differ in him, but are one good, he is so omnipotent in simple goodness that no reality is capable of harming the highest good. That is why God cannot corrupt or lie. Very well, that which does not exist does not of itself have a capacity to exist, but if something else is capable of making it be, in that sense it can exist — by the capacity of the other. (Trans. Anselm of Canterbury: The Major Works, ed. Davies & Evans)
To do evil is not capacity but incapacity. People, angels and humans, only do evil out of an incapacity to do good. God, in his plenitude of goodness, will never fall into evil because he will never fail. By his very nature, God is omnipotent. Evil itself bespeaks weakness.
Thus, God cannot will evil, because that statement itself is not spoken properly. No one, properly speaking, wills evil, since evil is a lack.