Objection 1. It would seem that a judge may pass sentence on a man who is not accused. For human justice is derived from Divine justice. Now God judges the sinner even though there be no accuser. Therefore it seems that a man may pass sentence of condemnation on a man even though there be no accuser.
Objection 2. Further, an accuser is required in judicial procedure in order that he may relate the crime to the judge. Now sometimes the crime may come to the judge's knowledge otherwise than by accusation; for instance, by denunciation, or by evil report, or through the judge himself being an eye-witness. Therefore a judge may condemn a man without there being an accuser.
Objection 3. Further, the deeds of holy persons are related in Holy Writ, as models of human conduct. Now Daniel was at the same time the accuser and the judge of the wicked ancients (Daniel 13). Therefore it is not contrary to justice for a man to condemn anyone as judge while being at the same time his accuser.
On the contrary, Ambrose in his commentary on 1 Corinthians 5:2, expounding the Apostle's sentence on the fornicator, says that "a judge should not condemn without an accuser, since our Lord did not banish Judas, who was a thief, yet was not accused."
I answer that, A judge is an interpreter of justice. Wherefore, as the Philosopher says (Ethic. v, 4), "men have recourse to a judge as to one who is the personification of justice." Now, as stated above (Question 58, Article 2), justice is not between a man and himself but between one man and another. Hence a judge must needs judge between two parties, which is the case when one is the prosecutor, and the other the defendant. Therefore in criminal cases the judge cannot sentence a man unless the latter has an accuser, according to Acts 25:16: "It is not the custom of the Romans to condemn any man, before that he who is accused have his accusers present, and have liberty to make his answer, to clear himself of the crimes" of which he is accused.
Reply to Objection 1. God, in judging man, takes the sinner's conscience as his accuser, according to Romans 2:15, "Their thoughts between themselves accusing, or also defending one another"; or again, He takes the evidence of the fact as regards the deed itself, according to Genesis 4:10, "The voice of thy brother's blood crieth to Me from the earth."
Reply to Objection 2. Public disgrace takes the place of an accuser. Hence a gloss on Genesis 4:10, "The voice of thy brother's blood," etc. says: "There is no need of an accuser when the crime committed is notorious." On a case of denunciation, as stated above (Question 33, Article 07), the amendment, not the punishment, of the sinner is intended: wherefore when a man is denounced for a sin, nothing is done against him, but for him, so that no accuser is required. The punishment that is inflicted is on account of his rebellion against the Church, and since this rebellion is manifest, it stands instead of an accuser. The fact that the judge himself was an eye-witness, does not authorize him to proceed to pass sentence, except according to the order of judicial procedure.
Reply to Objection 3. God, in judging man, proceeds from His own knowledge of the truth, whereas man does not, as stated above (Article 2). Hence a man cannot be accuser, witness and judge at the same time, as God is. Daniel was at once accuser and judge, because he was the executor of the sentence of God, by whose instinct he was moved, as stated above (1, ad 1).
Source: Summa Theologica (New Advent)