Objection 1. It would seem that every human action is good, and that none is evil. For Dionysius says (Div. Nom. iv) that evil acts not, save in virtue of the good. But no evil is done in virtue of the good. Therefore no action is evil.
Objection 2. Further, nothing acts except in so far as it is in act. Now a thing is evil, not according as it is in act, but according as its potentiality is void of act; whereas in so far as its potentiality is perfected by act, it is good, as stated in Metaph. ix, 9. Therefore nothing acts in so far as it is evil, but only according as it is good. Therefore every action is good, and none is evil.
Objection 3. Further, evil cannot be a cause, save accidentally, as Dionysius declares (Div. Nom. iv). But every action has some effect which is proper to it. Therefore no action is evil, but every action is good.
On the contrary, Our Lord said (John 3:20): "Every one that doth evil, hateth the light." Therefore some actions of man are evil.
I answer that, We must speak of good and evil in actions as of good and evil in things: because such as everything is, such is the act that it produces. Now in things, each one has so much good as it has being: since good and being are convertible, as was stated in the I, 5, 1,3. But God alone has the whole plenitude of His Being in a certain unity: whereas every other thing has its proper fulness of being in a certain multiplicity. Wherefore it happens with some things, that they have being in some respect, and yet they are lacking in the fulness of being due to them. Thus the fulness of human being requires a compound of soul and body, having all the powers and instruments of knowledge and movement: wherefore if any man be lacking in any of these, he is lacking in something due to the fulness of his being. So that as much as he has of being, so much has he of goodness: while so far as he is lacking in goodness, and is said to be evil: thus a blind man is possessed of goodness inasmuch as he lives; and of evil, inasmuch as he lacks sight. That, however, which has nothing of being or goodness, could not be said to be either evil or good. But since this same fulness of being is of the very essence of good, if a thing be lacking in its due fulness of being, it is not said to be good simply, but in a certain respect, inasmuch as it is a being; although it can be called a being simply, and a non-being in a certain respect, as was stated in the I, 5, 1, ad 1. We must therefore say that every action has goodness, in so far as it has being; whereas it is lacking in goodness, in so far as it is lacking in something that is due to its fulness of being; and thus it is said to be evil: for instance if it lacks the quantity determined by reason, or its due place, or something of the kind.
Reply to Objection 1. Evil acts in virtue of deficient goodness. For it there were nothing of good there, there would be neither being nor possibility of action. On the other hand if good were not deficient, there would be no evil. Consequently the action done is a deficient good, which is good in a certain respect, but simply evil.
Reply to Objection 2. Nothing hinders a thing from being in act in a certain respect, so that it can act; and in a certain respect deficient in act, so as to cause a deficient act. Thus a blind man has in act the power of walking, whereby he is able to walk; but inasmuch as he is deprived of sight he suffers a defect in walking by stumbling when he walks.
Reply to Objection 3. An evil action can have a proper effect, according to the goodness and being that it has. Thus adultery is the cause of human generation, inasmuch as it implies union of male and female, but not inasmuch as it lacks the order of reason.
Source: Summa Theologica (New Advent)