"Your hand will find out all of your enemies" (21:8). Who is you here, and how can a song of praise say what 21:9-10 say?

The psalm doesn't name the you of 21:8-12. In 21:1-6 you is the LORD: "The king rejoices in your strength, LORD!" (21:1). In 21:7 the psalm speaks of the king and of the LORD, not to either: "For the king trusts in the LORD" (21:7). Then "Your hand will find out all of your enemies. Your right hand will find out those who hate you" (21:8), and one verse has both you and the LORD: "You will make them as a fiery furnace in the time of your anger. The LORD will swallow them up in his wrath" (21:9) (tap furnace). The last verse speaks to the LORD again: "Be exalted, LORD, in your strength" (21:13). What the verses do say plainly is whose the wrath is, "The LORD will swallow them up in his wrath" (21:9), and why: "For they intended evil against you. They plotted evil against you which cannot succeed" (21:11). The psalm doesn't name the enemies, and these notes apply its words to no people today. Psalm 21 is not the only psalm that speaks of God acting against enemies, and Christians read these words in more than one way. The chapter on Psalm 109 sets out the main readings, each resting on its verses, and chooses none. These notes don't choose either.