Objection 1. It would seem that sin is not aggravated by reason of the condition of the person against whom it is committed. For if this were the case a sin would be aggravated chiefly by being committed against a just and holy man. But this does not aggravate a sin: because a virtuous man who bears a wrong with equanimity is less harmed by the wrong done him, than others, who, through being scandalized, are also hurt inwardly. Therefore the condition of the person against whom a sin is committed does not aggravate the sin.
Objection 2. Further, if the condition of the person aggravated the sin, this would be still more the case if the person be near of kin, because, as Cicero says (Paradox. iii): "The man who kills his slave sins once: he that takes his father's life sins many times." But the kinship of a person sinned against does not apparently aggravate a sin, because every man is most akin to himself; and yet it is less grievous to harm oneself than another, e.g. to kill one's own, than another's horse, as the Philosopher declares (Ethic. v, 11). Therefore kinship of the person sinned against does not aggravate the sin.
Objection 3. Further, the condition of the person who sins aggravates a sin chiefly on account of his position or knowledge, according to Wisdom 6:7: "The mighty shall be mightily tormented," and Luke 12:47: "The servant who knew the will of his lord... and did it not... shall be beaten with many stripes." Therefore, in like manner, on the part of the person sinned against, the sin is made more grievous by reason of his position and knowledge. But, apparently, it is not a more grievous sin to inflict an injury on a rich and powerful person than on a poor man, since "there is no respect of persons with God" (Colossians 3:25), according to Whose judgment the gravity of a sin is measured. Therefore the condition of the person sinned against does not aggravate the sin.
On the contrary, Holy Writ censures especially those sins that are committed against the servants of God. Thus it is written (1 Kings 19:14): "They have destroyed Thy altars, they have slain Thy prophets with the sword." Moreover much blame is attached to the sin committed by a man against those who are akin to him, according to Micah 7:6: "the son dishonoreth the father, and the daughter riseth up against her mother." Furthermore sins committed against persons of rank are expressly condemned: thus it is written (Job 34:18): "Who saith to the king: 'Thou art an apostate'; who calleth rulers ungodly." Therefore the condition of the person sinned against aggravates the sin.
I answer that, The person sinned against is, in a manner, the object of the sin. Now it has been stated above (Article 3) that the primary gravity of a sin is derived from its object; so that a sin is deemed to be so much the more grave, as its object is a more principal end. But the principal ends of human acts are God, man himself, and his neighbor: for whatever we do, it is on account of one of these that we do it; although one of them is subordinate to the other. Therefore the greater or lesser gravity of a sin, in respect of the person sinned against, may be considered on the part of these three.
First, on the part of God, to Whom man is the more closely united, as he is more virtuous or more sacred to God: so that an injury inflicted on such a person redounds on to God according to Zechariah 2:8: "He that toucheth you, toucheth the apple of My eye." Wherefore a sin is the more grievous, according as it is committed against a person more closely united to God by reason of personal sanctity, or official station. On the part of man himself, it is evident that he sins all the more grievously, according as the person against whom he sins, is more united to him, either through natural affinity or kindness received or any other bond; because he seems to sin against himself rather than the other, and, for this very reason, sins all the more grievously, according to Sirach 14:5: "He that is evil to himself, to whom will he be good?" On the part of his neighbor, a man sins the more grievously, according as his sin affects more persons: so that a sin committed against a public personage, e.g. a sovereign prince who stands in the place of the whole people, is more grievous than a sin committed against a private person; hence it is expressly prohibited (Exodus 22:28): "The prince of thy people thou shalt not curse." In like manner it would seem that an injury done to a person of prominence, is all the more grave, on account of the scandal and the disturbance it would cause among many people.
Reply to Objection 1. He who inflicts an injury on a virtuous person, so far as he is concerned, disturbs him internally and externally; but that the latter is not disturbed internally is due to his goodness, which does not extenuate the sin of the injurer.
Reply to Objection 2. The injury which a man inflicts on himself in those things which are subject to the dominion of his will, for instance his possessions, is less sinful than if it were inflicted on another, because he does it of his own will; but in those things that are not subject to the dominion of his will, such as natural and spiritual goods, it is a graver sin to inflict an injury on oneself: for it is more grievous for a man to kill himself than another. Since, however, things belonging to our neighbor are not subject to the dominion of our will, the argument fails to prove, in respect of injuries done to such like things, that it is less grievous to sin in their regard, unless indeed our neighbor be willing, or give his approval.
Reply to Objection 3. There is no respect for persons if God punishes more severely those who sin against a person of higher rank; for this is done because such an injury redounds to the harm of many.
Source: Summa Theologica (New Advent)