Since “good” means perfection, “evil” means the lack of perfection. If something lacks that which, by nature, it ought to have, we call such privations evil.
Privation is essential to the definition of evil. Nature is either act, potency, or a composite. That which is in act is by definition perfect and good inasmuch as it is in act, potency is by definition ordered to act and therefore must have good within its notion. As that which can receive act, potency desires act, which is, as we stated previously, good. From this it follows that no being desires evil as such and, more fundamentally, no being can be evil by nature.
Nature seeks completion and perfection and so seeks act. This fulfillment must consist in goodness since goodness is simply the completion of nature and its operations. In relation to existence, nature is a potency which seeks act in existence to the fullest extent possible. Existence, as act, is the good of nature and therefore non-existence, the negation of being, is an evil.
When seen in this light, the maxim “Do good and avoid evil” is not a prescription from some authority that may be followed or disobeyed at the whim of a creature. It is the fundamental law of both being and action.