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...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.