This psalm is ascribed to David, and there is nothing in the psalm to make us doubt the correctness of the title. Kimchi supposes that it refers to the enemies of David in the time of Saul. Grotius and Knapp suppose that it refers to Ahithophel; Dathe, to Shimei; DeWette, that it refers to national foes at a later period than the time of David. It is impossible now to ascertain the occasion on which it was composed. It would seem to have been one of the most trying in the life of David, when his enemies were most bitter against him. It is one of the "imprecatory" psalms, and one which is as difficult to reconcile with a kind and forgiving spirit as any other in the book.
In the New Testament Acts 1:20 a part of the psalm is applied to Judas the traitor, but without its being necessary to conclude that it had any original reference to him. The conduct of Judas was like the conduct of the enemy of David; the language used in the one case might be properly used in the other.
The psalm consists of three parts:
I. A description of the enemies of the psalmist Psalm 109:1-5, as
(a) deceitful and lying;
(b) as using words of hatred;
(c) as fighting against him without cause;
(d) as returning evil for good, and hatred for love.
From this it would seem that the persons referred to were some who had been closely connected with the author; who had received important benefits from him; who had been the subjects of his prayer; and who pursued him from mere malice.
II. A prayer for the punishment of those who had thus wronged him - referring particularly to some one person who had been prominent, or who had instigated others, imploring the infliction of just punishment on him as if he were alone responsible, Psalm 109:6-20. It is in this part of the psalm that the principal difficulty in the interpretation consists, as this is made up of severe and apparently harsh and revengeful imprecations. All is in fact invoked on him that any man could ever desire to see inflicted on an enemy.
III. A prayer for the sufferer's own deliverance, with a promise of thanksgiving, Psalm 109:21-31. The psalmist here describes his miserable and suffering condition, and prays that God would interpose - expressing a willingness to suffer anything at the hand of man if God would be his friend - a willingness that they should continue to "curse," if God would "bless." As the result of all, he says that he would find delight in praise - in the public acknowledgment of the goodness of God.
On the phrase in the title, "To the chief Musician," see the notes at the title to Psalm 4:1-8,

The psalmist speaks against his inveterate enemies, Psalm 109:1-5. He prays against them, and denounces God's judgments, Psalm 109:6-15. The reason on which this is grounded, Psalm 109:16-20. He prays for his own safety and salvation, using many arguments to induce God to have mercy upon him, Psalm 109:21-31.
The title of this Psalm, To the chief Musician, A Psalm of David, has already often occurred, and on it the Versions offer nothing new. The Syriac says it is "a Psalm of David, when the people, without his knowledge, made Absalom king; on which account he was slain: but to us (Christians) he details the passion of Christ." That it contains a prophecy against Judas and the enemies of our Lord, is evident from Acts 1:20. Probably, in its primary meaning, (for such a meaning it certainly has), it may refer to Ahithophel. The execrations in it should be rendered in the future tense, as they are mere prophetic denunciations of God's displeasure against sinners. Taken in this light, it cannot be a stumbling-block to any person. God has a right to denounce those judgments which he will inflict on the workers of iniquity. But perhaps the whole may be the execrations of David's enemies against himself. See on Psalm 109:20 (note). Ahithophel, who gave evil counsel against David, and being frustrated hanged himself, was no mean prototype of Judas the traitor; it was probably on this account that St. Peter, Acts 1:20, applied it to the case of Judas, as a prophetic declaration concerning him, or at least a subject that might be accommodated to his case.

INTRODUCTION TO PSALM 109
To the chief Musician, A Psalm of David. This psalm was written by David, under the inspiration of the Holy Spirit, concerning Judas the betrayer of Christ, as is certain from Acts 1:16 hence it is used to be called by the ancients the Iscariotic psalm. Whether the occasion of it was the rebellion of Absalom, as some, or the persecution of Saul, as Kimchi; and whoever David might have in view particularly, whether Ahithophel, or Doeg the Edomite, as is most likely; yet it is evident that the Holy Ghost foresaw the sin of Judas, and prophesies of that, and of the ruin and misery that should come upon him; for the imprecations in this psalm are no other than predictions of future events, and so are not to be drawn into an example by men; nor do they breathe out anything contrary to the spirit of Christianity, but are proofs of it, since what is here predicted has been exactly accomplished. The title in the Syriac version is,
"a psalm of David when they created Absalom king without his knowledge, and for this cause he was slain; but to us it expounds the sufferings of the Christ of God;''
and indeed he is the person that is all along speaking in this psalm.

(Psalm 109:1-5) David complains of his enemies.
(Psalm 109:6-20) He prophesies their destruction.
(Psalm 109:21-31) Prayers and praises.

Imprecation upon the Curser Who Prefers the Curse to the Blessing
The אודה, corresponding like an echo to the הודו of Ps 107, is also found here in Psalm 109:30. But Psalm 109 is most closely related to Ps 69. Anger concerning the ungodly who requite love with ingratitude, who persecute innocence and desire the curse instead of the blessing, has here reached its utmost bound. The imprecations are not, however, directed against a multitude as in Ps 69, but their whole current is turned against one person. Is this Doeg the Edomite, or Cush the Benjamite? We do not know. The marks of Jeremiah's hand, which raised a doubt about the לדוד of Ps 69, are wanting here; and if the development of the thoughts appears too diffuse and overloaded to be suited to David, and also many expressions (as the inflected מעט in Psalm 109:8, the נכאה, which is explained by the Syriac, in Psalm 109:16, and the half-passive חלל in Psalm 109:22) look as though they belong to the later period of the language, yet we feel on the other hand the absence of any certain echoes of older models. For in the parallels Psalm 109:6, cf. Zac 3:1, and Psalm 109:18, Psalm 109:29, cf. Isaiah 59:17, it is surely not the mutual relationship but the priority that is doubtful; Psalm 109:22, however, in relation to Psalm 55:5 (cf. Psalm 109:4 with Psalm 55:5) is a variation such as is also allowable in one and the same poet (e.g., in the refrains). The anathemas that are here poured forth more extensively than anywhere else speak in favour of David, or at least of his situation. They are explained by the depth of David's consciousness that he is the anointed of Jahve, and by his contemplation of himself in Christ. The persecution of David was a sin not only against David himself, but also against the Christ in him; and because Christ is in David, the outbursts of the Old Testament wrathful spirit take the prophetic form, so that this Psalm also, like Ps 22 and Ps 69, is a typically prophetic Psalm, inasmuch as the utterance of the type concerning himself is carried by the Spirit of prophecy beyond himself, and thus the ara' is raised to the προφητεία ἐν εἴδει ἀρᾶς (Chrysostom). These imprecations are not, however, appropriate in the mouth of the suffering Saviour. It is not the spirit of Zion but of Sinai which here speaks out of the mouth of David; the spirit of Elias, which, according to Luke 9:55, is not the spirit of the New Testament. This wrathful spirit is overpowered in the New Testament by the spirit of love. But these anathemas are still not on this account so many beatings of the air. There is in them a divine energy, as in the blessing and cursing of every man who is united to God, and more especially of a man whose temper of mind is such as David's. They possess the same power as the prophetical threatenings, and in this sense they are regarded in the New Testament as fulfilled in the son of perdition (John 17:12). To the generation of the time of Jesus they were a deterrent warning not to offend against the Holy One of God, and this Psalmus Ischarioticus (Acts 1:20) will ever be such a mirror of warning to the enemies and persecutors of Christ and His Church.