A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
psa 9:1Upon Muthlabben, or, after the manner according to "death to the Son," by which some song was known, to whose air or melody the musician is directed to perform this Psalm. This mode of denoting a song by some prominent word or words is still common (compare Psa 22:1). The Psalmist praises God for deliverance from his enemies and celebrates the divine government, for providing security to God's people and punishment to the wicked. Thus encouraging himself, he prays for new occasions to recount God's mercies, and confident of His continued judgment on the wicked and vindication of the oppressed, he implores a prompt and efficient manifestation of the divine sovereignty. (Psa. 9:1-20)
Heartfelt gratitude will find utterance.
psa 9:3When . . . are turned back--It is the result of God's power alone. He, as a righteous Judge (Psa 7:11), vindicates His people. He rebukes by acts as well as words (Psa 6:1; Psa 18:15), and so effectually as to destroy the names of nations as well as persons.
psa 9:6Literally, "As to the enemy finished are his ruins for ever. Thou [God] hast destroyed," &c. (Sa1 15:3, Sa1 15:7; Sa1 27:8-9). The wicked are utterly undone. Their ruins shall never be repaired.
psa 9:7God's eternal possession of a throne of justice is contrasted with the ruin of the wicked.
psa 9:9The oppressed, and all who know Him (Psa 5:3; Psa 7:1), find Him a sure refuge.
psa 9:11(Compare Psa 2:6; Psa 3:4).
psa 9:12for blood--that is, murders (Psa 5:6), including all the oppressions of His people.
maketh inquisition--(compare Gen 9:5). He will avenge their cause.
psa 9:13gates--or, "regions."
of death--Gates being the entrance is put for the bounds.
psa 9:14gates . . . Zion--The enclosure of the city (compare Psa 48:12; Isa 23:12), or, church, as denoted by this phrase contrasted with that of death, carries out the idea of exaltation as well as deliverance. Signal favors should lead us to render signal and public thanks.
psa 9:15The undesigned results of the devices of the wicked prove them to be of God's overruling or ordering, especially when those results are destructive to the wicked themselves.
psa 9:16Higgaion--means "meditation," and, combined with Selah, seems to denote a pause of unusual solemnity and emphasis (compare Psa 3:2). Though Selah occurs seventy-three times, this is the only case in which Higgaion is found. In the view which is given here of the retribution on the wicked as an instance of God's wise and holy ordering, we may well pause in adoring wonder and faith.
psa 9:17shall be turned--or, "shall turn," retreating under God's vengeance, and driven by Him to the extreme of destruction, even hell itself. Those who forget God are classed with the depraved and openly profane.
psa 9:18(Compare Psa 13:1-6).
the needy--literally, "poor," as deprived of anything; hence miserable.
expectation of the poor--or, "meek," "humble," made so by affliction.
psa 9:19Arise--(compare Psa 4:7).
let not man-- (Psa 8:4).
let . . . be judged--and of course condemned.
psa 9:20By their effectual subjection, make them to realize their frail nature (Psa 8:4), and deter them from all conceit and future rebellion.