Objection 1. It seems that a human action is not right or sinful, in so far as it is good or evil. For "monsters are the sins of nature" (Phys. ii, 8). But monsters are not actions, but things engendered outside the order of nature. Now things that are produced according to art and reason imitate those that are produced according to nature (Phys. ii, 8). Therefore an action is not sinful by reason of its being inordinate and evil.
Objection 2. Further, sin, as stated in Phys. ii, 8 occurs in nature and art, when the end intended by nature or art is not attained. But the goodness or malice of a human action depends, before all, on the intention of the end, and on its achievement. Therefore it seems that the malice of an action does not make it sinful.
Objection 3. Further, if the malice of an action makes it sinful, it follows that wherever there is evil, there is sin. But this is false: since punishment is not a sin, although it is an evil. Therefore an action is not sinful by reason of its being evil.
On the contrary, As shown above (Question 19, Article 4), the goodness of a human action depends principally on the Eternal Law: and consequently its malice consists in its being in disaccord with the Eternal Law. But this is the very nature of sin; for Augustine says (Contra Faust. xxii, 27) that "sin is a word, deed, or desire, in opposition to the Eternal Law." Therefore a human action is sinful by reason of its being evil.
I answer that, Evil is more comprehensive than sin, as also is good than right. For every privation of good, in whatever subject, is an evil: whereas sin consists properly in an action done for a certain end, and lacking due order to that end. Now the due order to an end is measured by some rule. In things that act according to nature, this rule is the natural force that inclines them to that end. When therefore an action proceeds from a natural force, in accord with the natural inclination to an end, then the action is said to be right: since the mean does not exceed its limits, viz. the action does not swerve from the order of its active principle to the end. But when an action strays from this rectitude, it comes under the notion of sin.
Now in those things that are done by the will, the proximate rule is the human reason, while the supreme rule is the Eternal Law. When, therefore, a human action tends to the end, according to the order of reason and of the Eternal Law, then that action is right: but when it turns aside from that rectitude, then it is said to be a sin. Now it is evident from what has been said (19, 3,4) that every voluntary action that turns aside from the order of reason and of the Eternal Law, is evil, and that every good action is in accord with reason and the Eternal Law. Hence it follows that a human action is right or sinful by reason of its being good or evil.
Reply to Objection 1. Monsters are called sins, inasmuch as they result from a sin in nature's action.
Reply to Objection 2. The end is twofold; the last end, and the proximate end. In the sin of nature, the action does indeed fail in respect of the last end, which is the perfection of the thing generated; but it does not fail in respect of any proximate end whatever; since when nature works it forms something. In like manner, the sin of the will always fails as regards the last end intended, because no voluntary evil action can be ordained to happiness, which is the last end: and yet it does not fail in respect of some proximate end: intended and achieved by the will. Wherefore also, since the very intention of this end is ordained to the last end, this same intention may be right or sinful.
Reply to Objection 3. Each thing is ordained to its end by its action: and therefore sin, which consists in straying from the order to the end, consists properly in an action. On the other hand, punishment regards the person of the sinner, as was stated in the I, 48, 5, ad 4; 6, ad 3.
Source: Summa Theologica (New Advent)