Objection 1. It would seem that a sin is not aggravated by reason of its causing more harm. Because the harm done is an issue consequent to the sinful act. But the issue of an act does not add to its goodness or malice, as stated above (Question 20, Article 5). Therefore a sin is not aggravated on account of its causing more harm.
Objection 2. Further, harm is inflicted by sins against our neighbor. Because no one wishes to harm himself: and no one can harm God, according to Job 35:6-8: "If thy iniquities be multiplied, what shalt thou do against Him?... Thy wickedness may hurt a man that is like thee." If, therefore, sins were aggravated through causing more harm, it would follow that sins against our neighbor are more grievous than sins against God or oneself.
Objection 3. Further, greater harm is inflicted on a man by depriving him of the life of grace, than by taking away his natural life; because the life of grace is better than the life of nature, so far that man ought to despise his natural life lest he lose the life of grace. Now, speaking absolutely, a man who leads a woman to commit fornication deprives her of the life of grace by leading her into mortal sin. If therefore a sin were more grievous on account of its causing a greater harm, it would follow that fornication, absolutely speaking, is a more grievous sin than murder, which is evidently untrue. Therefore a sin is not more grievous on account of its causing a greater harm.
On the contrary, Augustine says (De Lib. Arb. iii, 14): "Since vice is contrary to nature, a vice is the more grievous according as it diminishes the integrity of nature." Now the diminution of the integrity of nature is a harm. Therefore a sin is graver according as it does more harm.
I answer that, Harm may bear a threefold relation to sin. Because sometimes the harm resulting from a sin is foreseen and intended, as when a man does something with a mind to harm another, e.g. a murderer or a thief. In this case the quantity of harm aggravates the sin directly, because then the harm is the direct object of the sin. Sometimes the harm is foreseen, but not intended; for instance, when a man takes a short cut through a field, the result being that he knowingly injures the growing crops, although his intention is not to do this harm, but to commit fornication. In this case again the quantity of the harm done aggravates the sin; indirectly, however, in so far, to wit, as it is owing to his will being strongly inclined to sin, that a man does not forbear from doing, to himself or to another, a harm which he would not wish simply. Sometimes, however, the harm is neither foreseen nor intended: and then if this harm is connected with the sin accidentally, it does not aggravate the sin directly; but, on account of his neglecting to consider the harm that might ensue, a man is deemed punishable for the evil results of his action if it be unlawful. If, on the other hand, the harm follow directly from the sinful act, although it be neither foreseen nor intended, it aggravates the sin directly, because whatever is directly consequent to a sin, belongs, in a manner, to the very species of that sin: for instance, if a man is a notorious fornicator, the result is that many are scandalized; and although such was not his intention, nor was it perhaps foreseen by him, yet it aggravates his sin directly.
But this does not seem to apply to penal harm, which the sinner himself incurs. Such like harm, if accidentally connected with the sinful act, and if neither foreseen nor intended, does not aggravate a sin, nor does it correspond with the gravity of the sin: for instance, if a man in running to slay, slips and hurts his foot. If, on the other hand, this harm is directly consequent to the sinful act, although perhaps it be neither foreseen nor intended, then greater harm does not make greater sin, but, on the contrary, a graver sin calls for the infliction of a greater harm. Thus, an unbeliever who has heard nothing about the pains of hell, would suffer greater pain in hell for a sin of murder than for a sin of theft: but his sin is not aggravated on account of his neither intending nor foreseeing this, as it would be in the case of a believer, who, seemingly, sins more grievously in the very fact that he despises a greater punishment, that he may satisfy his desire to sin; but the gravity of this harm is caused by the sole gravity of sin.
Reply to Objection 1. As we have already stated (20, 5), in treating of the goodness and malice of external actions, the result of an action if foreseen and intended adds to the goodness and malice of an act.
Reply to Objection 2. Although the harm done aggravates a sin, it does not follow that this alone renders a sin more grievous: in fact, it is inordinateness which of itself aggravates a sin. Wherefore the harm itself that ensues aggravates a sin, in so far only as it renders the act more inordinate. Hence it does not follow, supposing harm to be inflicted chiefly by sins against our neighbor, that such sins are the most grievous, since a much greater inordinateness is to be found against which man commits against God, and in some which he commits against himself. Moreover we might say that although no man can do God any harm in His substance, yet he can endeavor to do so in things concerning Him, e.g. by destroying faith, by outraging holy things, which are most grievous sins. Again, a man sometimes knowingly and freely inflicts harm on himself, as in the case of suicide, though this be referred finally to some apparent good, for example, delivery from some anxiety.
Reply to Objection 3. This argument does not prove, for two reasons: first, because the murderer intends directly to do harm to his neighbors; whereas the fornicator who solicits the woman intends not to harm but pleasure; secondly, because murder is the direct and sufficient cause of bodily death; whereas no man can of himself be the sufficient cause of another's spiritual death, because no man dies spiritually except by sinning of his own will.
Source: Summa Theologica (New Advent)