Albert Barnes Commentary Psalms 149:7

"To execute vengeance upon the nations, And punishments upon the peoples;" — Psalms 149:7 (ASV)

To execute vengeance upon the heathen - To inflict punishment upon them as a recompense for their sins. The word pagan here means nations. The allusion is, doubtless, to those who had oppressed and injured the Hebrew people—perhaps referring to those who had destroyed the city and the temple at the time of the Babylonian captivity. They were now to receive the punishment due for the wrongs which they had done to the nation; a just recompense at the hand of God, and by the instrumentality of those whom they had wronged. Compare the notes at Psalms 137:7-9.

And punishments upon the people - The people of those lands. Those who had waged war with the Hebrew nation.