Because he forgot to show mercy The prophet comes now to show that he had good reason for desiring such awful and direful calamities to be inflicted upon his enemies, whose thirst for cruelty was insatiable, and who were transported with rage, no less cruel than obstinate, against the afflicted and poor man, persecuting him with as little scruple as if they were attacking a dead dog. Even philosophers look upon cruelty, directed against the helpless and miserable, as an act worthy only of a cowardly and grovelling nature; for it is between equals that envy is cherished. For this reason the prophet represents the malignity of his enemies as being bitter in persecuting him when he was in affliction and poverty. The expression, the sorrowful in heart, is still more emphatic. For there are persons who, notwithstanding of their afflictions, are puffed up with pride; and as this conduct is unreasonable and unnatural, these individuals incur the displeasure of the powerful. On the other hand, it would be a sign of desperate cruelty to treat with contempt the lowly and dejected in heart. Would not this be to fight with a shadow? This insatiable cruelty is still farther pointed out by the phrase, forgetting to show mercy; the meaning of which is, that the calamities, with which he beheld this guiltless and miserable man struggling, fail to excite his pity, so that, out of regard to the common lot of humanity, he should lay aside his savage disposition. In this passage, therefore, the contrast is equally balanced on the one side between such obstinate pride, and on the other, the strict and irrevocable judgment of God. And as David spoke only as he was moved by the Holy Spirit, this imprecation must be received as if God himself should thunder from his celestial throne. Thus, in the one case, by denouncing vengeance against the ungodly, he subdues and restrains our perverse inclinations, which might lead us to injure a fellow-creature; and on the other, by imparting comfort to us, he mitigates and moderates our sorrow, so that we patiently endure the ills which they inflict upon us. The wicked may for a time revel with impunity in the gratification of their lusts; but this threatening shows that it is no vain protection which God vouchsafes to the afflicted. But let the faithful conduct themselves meekly, that their humility and contrition of spirit may come up before God with acceptance. And as we cannot distinguish between the elect and the reprobate, it is our duty to pray for all who trouble us; to desire the salvation of all men; and even to be careful for the welfare of every individual. At the same time, if our hearts are pure and peaceful, this will not prevent us from freely appealing to God's judgment, that he may cut off the finally impenitent.

Because that he remembered not to show mercy - He had no compassion; he was severe, harsh, unjust, unfeeling.
But persecuted the poor and needy man - The man that was destitute of friends; that was a wanderer and a beggar. There were times in the life of David when this would be strictly and literally applicable to him.
That he might even slay the broken in heart - The man whose heart was crushed by sorrow - that he might put "the finishing stroke" to all, and send him to the grave. Whatever might have been the "feeling" which prompted to this prayer, or however difficult it may be to vindicate the psalmist's expression of feeling, there can be no doubt as to the propriety of inflicting punishment on such a man. The sufferings invoked are none too severe to be inflicted on a man who persecutes the poor and needy, and seeks so to multiply sorrows that the man already crushed and broken in heart shall sink to the grave.

Persecuted the poor and needy man - In the case of Jesus Christ all the dictates of justice and mercy were destroyed, and they persecuted this poor man unto death. They acted from a diabolical malice. On common principles, their opposition to Christ cannot be accounted for.

Because that (h) he remembered not to shew mercy, but persecuted the poor and needy man, that he might even slay the broken in heart.
(h) He shows that God plagues them in a strange way who show themselves cruel toward others.

Because that he remembered not to show mercy,.... As Judas did not; neither to the poor, whom he cared not for, John 12:6 nor to Christ, whom he betrayed with a kiss to his enemies: nor had these words of Christ any effect upon him, to move his pity and compassion, "Friend, wherefore art thou come?" Matthew 26:49 nor did the Jews show mercy to him: they were a merciless and hardhearted people; though mercy was one of the weightier matters of the law, this they omitted, Matthew 23:23, their want of compassion may be observed in the priest and Levite passing by the man wounded by thieves, Luke 10:30. Nor did they show any mercy to Christ, when they smote and buffeted him; nor did it move their pity when Pilate brought him forth with a crown of thorns on his head, and in a miserable condition, saying, "Behold the man"; but they cried out, "Crucify him, crucify him"; and gave him gall for his meat, and vinegar to drink; and mocked him when in all his miseries and agonies.
But persecuted the poor and needy man; Christ, who became poor for our sakes, and stood in need of the ministration of others to him, 2-Corinthians 8:9 and was poor in spirit, a man of sorrows, and acquainted with griefs; him Judas and the Jews persecuted to death, as follows:
that he might even slay the broken in heart; Christ, whose heart was broken with the reproach and cruel usage of men, Psalm 69:20, whose life the Jews sought to take away, and by means of Judas did.

Let God remember guilt, because he (the wicked) did not remember mercy.
poor and needy . . . broken in heart--that is, pious sufferer (Psalm 34:18; Psalm 35:10; Psalm 40:17).

He whom he persecuted with a thirst for blood, was, apart from this, a great sufferer, bowed down and poor and נכאה לבב, of terrified, confounded heart. lxx κατανενυγμένον (Jerome, compunctum); but the stem-word is not נכא (נכה), root נך, but כּאה, Syriac bā'ā', cogn. כּהה, to cause to come near, to meet. The verb, and more especially in Niph., is proved to be Hebrew by Daniel 11:30. Such an one who without anything else is of a terrified heart, inasmuch as he has been made to feel the wrath of God most keenly, this man has persecuted with a deadly hatred. He had experienced kindness (חסד) in a high degree, but he blotted out of his memory that which he had experienced, not for an instant imagining that he too on his part had to exercise חסד. The Poel מותת instead of המית points to the agonizing death (Isaiah 53:9, cf. Ezekiel 28:10 מותי) to which he exposes God's anointed. The fate of the shedder of blood is not expressed after the manner of a wish in Psalm 109:16-18, but in the historical form, as being the result that followed of inward necessity from the matter of fact of the course which he had himself determined upon. The verb בּוא seq. acc. signifies to surprise, suddenly attack any one, as in Isaiah 41:25. The three figures in Psalm 109:18 are climactic: he has clothed himself in cursing, he has drunk it in like water (Job 15:16; Job 34:7), it has penetrated even to the marrow of his bones, like the oily preparations which are rubbed in and penetrate to the bones.n In Psalm 109:19 the emphasis rests upon יעטּה and upon תּמיד. The summarizing Psalm 109:20 is the close of a strophe. פּעלּה, an earned reward, here punishment incurred, is especially frequent in Isaiah 40:1, e.g., Psalm 49:4; Psalm 40:10; it also occurs once even in the Tra, Leviticus 19:13. Those who answer the loving acts of the righteous with such malevolence in word and in deed commit a satanic sin for which there is no forgiveness. The curse is the fruit of their own choice and deed. Arnobius: Nota ex arbitrio evenisse ut nollet, propter haeresim, quae dicit Deum alios praedestinasse ad benedictionem, alios ad maledictionem.