7 He then begins to prophesy what they should receive for this very impiety; detailing their lot in such a manner, as if he wished its realization from a desire of revenge. Some not understanding this mode of predicting the future, under the appearance of wishing evil, suppose hatred to be returned for hatred, and an evil will for an evil will, since in truth it belongs to few to distinguish, in what way the punishment of the wicked pleases the accuser, who longs to satiate his enmity; and in how widely different a way it pleases the judge, who with a righteous mind punishes sins. For the former returns evil for evil: but the judge when he punishes does not return evil for evil, since he returns justice to the unjust; and what is just, is surely good. He therefore punishes not from delight in another's misery, which is evil for evil: but from love of justice, which is good for evil....
8. “Set thou an ungodly man to be ruler over him; and let Satan stand at his right hand”. Though the complaint had been before concerning many, the Psalm is now speaking of one....Since therefore he is here speaking of the traitor Judas, who, according to the Scripture in the Acts of the Apostles, was to be punished with the penalty due to him, what means, “set thou an ungodly man over him,” save him whom in the next verse he mentions by name, when he says, “and let Satan stand at his right hand”? He therefore who refused to be subject unto Christ, deserved this, that he should have the devil set over him, that is, that he should be subject unto the devil....For this reason also it is said of those who, preferring the pleasures of this world to God, styled the people blessed who have such and such things, “their right hand is a right hand of iniquity.”...
9. “When sentence is given upon him, let him be condemned, and let his prayer be turned into sin”. For prayer is not righteous except through Christ, whom he sold in his atrocious sin: but the prayer which is not made through Christ, not only cannot blot out sin, but is itself turned into sin. But it may be inquired on what occasion Judas could have so prayed, that his prayer was turned into sin. I suppose that before he betrayed the Lord, while he was thinking of betraying Him; for he could no longer pray through Christ. For after he betrayed Him, and repented of it, if he prayed through Christ, he would ask for pardon; if he asked for pardon, he would have hope; if he had hope, he would hope for mercy; if he hoped for mercy, he would not have hanged himself in despair....
10. “Let his days be few”. By “his days,” he meant the days of his apostleship, which were few; since before the Passion of our Lord, they were ended by his crime and death. And as if it were asked, What then shall become of that most sacred number twelve, within which our Lord willed, not without a meaning, to limit His twelve first Apostles? He at once adds, “and let another take his office.” As much as to say, let both himself be punished according to his desert, and let his number be filled up.
11. “Let his children be fatherless, and his wife a widow”. After his death, both his children were fatherless, and his wife a widow. “Let his children be vagabonds, and be carried away, and beg their bread”. By “vagabonds” he means, uncertain whither to go, destitute of all help. “Let them be driven from their habitations.” He here explains what he had said above, “Let them be carried away.” How all this happened to his wife and children, the following verses explain.
12. “Let the extortioner search out all his substance, and let the strangers spoil his labour”. “Let there be no man to help him”: that is, to guard his posterity; wherefore follows, “nor to have compassion on his fatherless children”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)