Thomas Aquinas, Compendium of Theology

Knowing that evil is the privation of a due perfection, we can easily understand how evil corrupts good; this it does to the extent that it is the privation of good. Thus blindness is said to corrupt sight because it is the privation of sight. However, evil does not completely corrupt good, because, as we remarked above, not only form, but also potency to form, is good; and potency is the subject of privation as well as of form. Therefore the subject of evil must be good, not in the sense that it is opposed to evil, but in the sense that it is a potency for the reception of evil. This brings out the fact that not every good can be the subject of evil, but only such a good as is in potency with respect to some perfection of which it can be deprived. Hence in beings which are exclusively act, or in which act cannot be separated from potency, there can, to this extent, be no evil.

As a result, nothing can be essentially evil, since evil must always have as its foundation some subject, distinct from it, that is good. And so there cannot be a being that is supremely evil, in the way that there is a being that is supremely good because it is essentially good.

Further, we see clearly that evil cannot be the object of desire, and that it cannot act except in virtue of the good connected with it. For only perfection and end are desirable; and the principle of action is form. However, since a particular perfection or form involves the privation of some other perfection or form, it can happen incidentally that privation or evil may be desired and may be the principle of some action; not precisely because of the evil, but because of the good connected with it. An example of what I here mean by "incidentally" is the musician who constructs a house, not in his capacity of musician, but in the capacity of being also a builder.

From this we may also infer that evil cannot be a first principle, for a principle per accidens is subsequent to a principle that is such per se.