Objection 1. It would seem that a man is not bound to accuse. For no man is excused on account of sin from fulfilling a Divine precept, since he would thus profit by his sin. Yet on account of sin some are disqualified from accusing, such as those who are excommunicate or of evil fame, or who are accused of grievous crimes and are not yet proved to be innocent [1 Timothy 1:5. Therefore a man is not bound by a Divine precept to accuse.
Objection 2. Further, every duty depends on charity which is "the end of the precept" [Can. Definimus, caus. iv, qu. 1; caus. vi, qu. 1: wherefore it is written (Romans 13:8): "Owe no man anything, but to love one another." Now that which belongs to charity is a duty that man owes to all both of high and of low degree, both superiors and inferiors. Since therefore subjects should not accuse their superiors, nor persons of lower degree, those of a higher degree, as shown in several chapters (Decret. II, qu. vii), it seems that it is no man's duty to accuse.
Objection 3. Further, no man is bound to act against the fidelity which he owes his friend; because he ought not to do to another what he would not have others do to him. Now to accuse anyone is sometimes contrary to the fidelity that one owes a friend; for it is written (Proverbs 11:13): "He that walketh deceitfully, revealeth secrets; but he that is faithful, concealeth the thing committed to him by his friend." Therefore a man is not bound to accuse.
On the contrary, It is written (Leviticus 5:1): "If any one sin, and hear the voice of one swearing, and is a witness either because he himself hath seen, or is privy to it: if he do not utter it, he shall bear his iniquity."
I answer that, As stated above (33, 6,7; 67, 3, ad 2), the difference between denunciation and accusation is that in denunciation we aim at a brother's amendment, whereas in accusation we intend the punishment of his crime. Now the punishments of this life are sought, not for their own sake, because this is not the final time of retribution, but in their character of medicine, conducing either to the amendment of the sinner, or to the good of the commonwealth whose calm is ensured by the punishment of evil-doers. The former of these is intended in denunciation, as stated, whereas the second regards properly accusation. Hence in the case of a crime that conduces to the injury of the commonwealth, a man is bound to accusation, provided he can offer sufficient proof, since it is the accuser's duty to prove: as, for example, when anyone's sin conduces to the bodily or spiritual corruption of the community. If, however, the sin be not such as to affect the community, or if he cannot offer sufficient proof, a man is not bound to attempt to accuse, since no man is bound to do what he cannot duly accomplish.
Reply to Objection 1. Nothing prevents a man being debarred by sin from doing what men are under an obligation to do: for instance from meriting eternal life, and from receiving the sacraments of the Church. Nor does a man profit by this: indeed it is a most grievous fault to fail to do what one is bound to do, since virtuous acts are perfections of man.
Reply to Objection 2. Subjects are debarred from accusing their superiors, "if it is not the affection of charity but their own wickedness that leads them to defame and disparage the conduct of their superiors" [Append. Grat. ad can. Sunt nonnulli, caus. ii, qu. 7 --or again if the subject who wishes to accuse his superior is himself guilty of crime [Decret. II, qu. vii, can. Praesumunt.]. Otherwise, provided they be in other respects qualified to accuse, it is lawful for subjects to accuse their superiors out of charity.
Reply to Objection 3. It is contrary to fidelity to make known secrets to the injury of a person; but not if they be revealed for the good of the community, which should always be preferred to a private good. Hence it is unlawful to receive any secret in detriment to the common good: and yet a thing is scarcely a secret when there are sufficient witnesses to prove it.
Source: Summa Theologica (New Advent)