Psalms 109:21

But do thou for me, O GOD the Lord, for thy name’s sake: because thy mercy is good, deliver thou me.

Psalm 109:21 stands within an inspired prayer and plea that declares God's justice and judgment upon the wicked enemies of the church. It is part of a prayer expressing judgment on those enemies, and part of a plea for divine justice against those who plot wickedness against the innocent. That declaration is predictive of coming tribulation, not an expression of personal revenge or a call to cruelty. It is not a personal prayer of David for himself, nor a request that his own enemies be cursed.

The punishments described in the psalm are the fruit of evil deeds returning upon the evildoers themselves. They come upon those whose lives are evil and who sought to bring those same punishments upon others. Those who plot wickedness against the innocent receive the fruit of their own deeds.