When he is judged, let him depart guilty Another imprecation is, that, being summoned to judgment, he might be punished without mercy, and that, though he humbly crave forgiveness, the judge should remain inexorable. This might with propriety be understood to relate not merely to his being judged at the bar of men, but also at the tribunal of God. But as it accords very well with the decisions awarded by an earthly judge, and as this is the commonly received interpretation, I have no wish to depart from it. There are two things which must be noticed here; that the wickedness of the wicked may be so palpable as to leave no room to escape from the execution of justice, and that all their entreaties for pardon may be disregarded. Accordingly, the Psalmist represents him as a condemned criminal leaving the presence of the judge, bearing the ignominy of the condemnation which he righteously merited, having his nefarious deeds disclosed and detected. With respect to the other interpretation which places the ungodly before God's judgment-seat, it by no means appears absurd to say that their prayers should be turned against them to sin, the more especially as we know that all their sacrifices are an abomination unto him. And by how much they themselves are filthy, by so much do all their plausible virtues become offensive and displeasing to God. But as the scope of the passage is in favor of that interpretation which applies it to earthly judges, I do not consider it necessary to insist farther upon this point.

When he shall be judged - When for his offences he shall be arraigned. The psalmist supposes that he "might" be put on trial; he seems to suppose that this "would be." Such wickedness could not always escape detection, and sooner or later he would be arrested and brought to trial. "When" this should occur, the psalmist prays that justice might be done; that he might be condemned, as he "ought" to be. Such a prayer could not in itself be wrong, for assuredly it cannot be proper for magistrates to pray that the wicked man may escape, or that they may themselves fail in the very object for which they are appointed. See the General Introduction, 6 (5) e. f.
And let his prayer become sin - Evidently his prayer in reference to his "trial" for crime; his prayer that he might be acquitted and discharged. Let it be seen in the result that such a prayer was wrong; that it was, in fact, a prayer for the discharge of a bad man - a man who ought to be punished. Let it be seen to be what a prayer would be if offered for a murderer, or violator of the law - a prayer that he might escape or not be punished. All must see that such a prayer would be wrong, or would be a "sin;" and so, in his own case, it would be equally true that a prayer "for his own escape" would be "sin." The psalmist asks that, by the result of the trial, such a prayer might be "seen" to be in fact a prayer "for the" protection and escape of a "bad man." A just sentence in the case would demonstrate this; and this is what the psalmist prays for.

Let him be condemned - יצא רשע yetse rasha. "Let him come out a wicked man;" that is let his wickedness be made manifest.
Let his prayer become sin - Thus paraphrased by Calmet: "Let him be accused, convicted, and condemned, and let the defense which he brings for his justification only serve to deepen his guilt, and hasten his condemnation." I once more apprise the reader, that if these are not the words of David's enemies against himself, (see on Psalm 109:20 (note)), they are prophetic denunciations against a rebellious and apostate person or people, hardened in crime, and refusing to return to God.

When he shall be judged, let him be condemned: and let his (d) prayer become sin.
(d) As to the elect all things turn to their profit, so to the reprobate, even those things that are good, turn to their damnation.

When he shall be judged, let him be condemned,.... When he shall be arraigned at the bar of his own conscience, and be charged with the sin of which he is guilty, let conscience, which is as a thousand witnesses, rise up against him, and condemn him; so it did Judas, Matthew 26:1, or when he shall stand before the judgment seat of Christ at the last day,
let him go out a wicked, or a guilty or condemned man (z); let him hear the awful sentence, "go, thou cursed, into everlasting fire": and let him go out immediately from the presence of the Judge into eternal punishment, the condemnation of the devil: so Judas is said to go to his own place, Acts 1:25.
And let his prayer become sin, let it be fruitless and in vain; and so far from being heard, let it he treated as an abomination; let it be considered as an aggravation of his crime, as Haman's was, Esther 7:7, let his prayer being without faith in the blood of Christ, be reckoned sinful, as it was; let his cries, and tears, and repentance issue in desperation, and that in sin, as it did in destroying himself, Matthew 27:5.
(z) "exeat impius", Pagninus, Montanus, Vatablus, De Dieu, Gejerus; "damnatus", Junius & Tremellius; "condemnatus", Cocceius.

The condemnation is aggravated when prayer for relief is treated as a sin.

Sin - Because it is not from his heart.