10 Execute mercy to the wicked, not as being wicked. Do not receive the wicked, in so far forth as he is wicked: that is, do not receive him as if from inclination towards and love for his iniquity. For it is forbidden to give unto a sinner, and to receive sinners. Yet how is this, “Give unto every man that asks of you”? And this, “if your enemy hunger, feed him”? This is seemingly contradictory: but it is opened to those who knock in the name of Christ, and will be clear unto those who seek. “Help not a sinner:” and, “give not to the ungodly;” and yet, “give unto every man that asks of you.” But it is a sinner who asks of me. Give, not as unto a sinner. When do you give as unto a sinner? When that which makes him a sinner, pleases you so that you give....Let those who give to a man who fights with wild beasts, tell me why they give? Why does he give to this man? He loves that in him, in which consists his greatest sin; this he feeds, this he clothes in him, wickedness itself, made public by all witnessing it. Why does the man give, who gives to actors, or to charioteers, or to courtesans? Do not these very persons give to human beings? But it is not the nature of God's work that they attend to, but the iniquity of the human work....When therefore you give, you give to infamy, not to bravery. As then he who gives to the fighter of beasts, gives not to the man, but to a most infamous profession; for if he were only a man, and not a fighter of beasts, you would not give; you honour him in vice, not nature: so on the other hand, if you give to the righteous, if you give to the prophet, if you give to the disciple of Christ anything of which he is in want, without thinking that he is Christ's disciple, that he is God's minister, that he is God's steward; but art thinking in that case of some temporal advantage, for instance, that when perchance he shall be needful to your cause, he may be bought for you, because you have given him something; you have no more given to the righteous, if you have thus given, than he gave to the man, when he gave to the beast-fighter. The matter, then, most beloved, is quite open to us, and I conceive, that although it was obscure, it is now clear. It was to this that the Lord bound you, when He said, “He who has received the righteous man.” That were enough. But as the righteous may be received with another intention,...He says, “He who receives a righteous man in the name of a righteous man:” that is, receiving him in consideration of his righteousness:...that is, because he is Christ's disciple, because he is a steward of the Mystery: “Verily I say unto you, he shall in no wise lose his reward.” So understand, he who receives a sinner in the name of a sinner shall lose his reward.
11....On this account therefore be merciful without fear, extend love even unto your enemies: punish those who chance to belong to your government, restrain them with affection, with charity, in regard to their eternal salvation; lest while you spare the flesh, the soul perish. Do this: and though thou have to endure many, over whom you can not exercise discipline, because you have no lawful authority over them; bear their injuries; be without apprehension. He will show mercy unto you if you shall have been merciful: you shall be merciful, without the injuries you suffer losing their punishment; “To Me belongs vengeance, I will repay,” says the Lord.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)