The Joy Of The Righteous At God's Intervention Which Demonstrates That Righteousness Will Prevail (Psalms 58:10-11).
The Psalm ends with the assurance that there is a God Who judges in the earth (or ‘land'), which will be made known to the righteous by His acts of vengeance on their behalf, in accordance with what has previously been described.
‘The righteous will rejoice when he sees the vengeance,
He will wash his feet in the blood of the wicked,'
When the righteous (those who are responsive to God's covenant of grace) see the unrighteous rendered harmless or swept away they will rejoice, not vindictively as Proverbs 24:17-18 makes clear, but because it means that righteousness has triumphed. The righteous are warned against seeking vengeance with the assurance that they can leave it in God's hands. “Vengeance belongs to Me,” says YHWH, “I will repay” (Romans 12:19; Hebrews 10:30; Psalms 94:1; Deuteronomy 32:35). But they can only rejoice when God finally does deal with unrighteousness. The picture of washing the feet in blood comes from the battlefield. The idea is not that the righteous choose to use the blood to wash in, but that they will be unable to avoid it, because God's judgment has made it inevitable. The thought is that judgment has come on the unrighteous and they have been totally defeated. Compare Revelation 14:20, and see Isaiah 63:2-3.
So that men will say, “Truly there is a reward for the righteous,
Truly there is a God who judges in the earth.'
The reason for the rejoicing of the righteous is now made clear. It is because it brings home to them that righteousness is finally rewarded, and that there truly is a God Who judges the earth.