Material substances are specified by their forms, and, in a similar way, moral creatures are specified by the end that is the proper object of the will. Just as the possession of one substantial form by a material substance entails the privation of other forms within the substance, moral creatures possess an end to the exclusion of others. Given that things are given ends by nature, we can say that the privation of proper form or perfection constitutes evil. Moral creatures are within this category. Their actions are evil because they entail the privation of the correct end of the will.
Evil, as such, is the privation of perfect being. The completion or perfection of a nature is the actualization of a potency and is what we first call "good." Notice that it is not only act that is called good but also potency, and potency is the subject of privation as well as act. Even a potency for evil must be, in a sense, good. In the case of God, who is pure act with no admixture of potency, we can say that His Goodness cannot possibly be the subject of some evil because good can only be the subject of evil inasmuch as it is in potency to some act. In other words, because a potency is a potency for some perfection it may also be deprived of the same perfection and is therefore what we call privation or evil. As pure act, none of this can be said of God. He is simply "outside" all potency and privation.
Further, we can say that evil can only be a principle per accidens. Only perfection is that which is desired by the will, but since the presence of one end excludes the presence of another, which is what constitutes the privation, privation and therefore evil maybe accidentally sought by the will. Evil as such cannot be a first or per se principle.