Charles H. Spurgeon on Psalms 109:7

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. —Albert Barnes.