Albert Barnes Commentary Psalms 109:20

"This is the reward of mine adversaries from Jehovah, And of them that speak evil against my soul." — Psalms 109:20 (ASV)

Let this be the reward of mine adversaries from the Lord,... - The word rendered “reward” usually means work, labor, occupation, or business; then, what one earns by his work—reward, recompense (Leviticus 19:13). The meaning here is, let them constantly receive these things which I have prayed for (Psalms 109:6–19); let them be constantly treated in this manner. This is a summing up of his entire wish—his whole desire. It cannot be proved that they did “not deserve” all this. It cannot be shown that if all this came upon them at the hand of God, it would be unjust. It cannot be denied that such things as these—either singly, in groups, or in succession—do actually come upon wicked people. And the prayer in this case may have been merely that justice might be done.

Still, as before remarked, it is not easy wholly to vindicate the expressed feelings of the psalmist. See the notes at Psalm 109:10.