What does Psalm 139:20 mean?

In this verse David levels two charges against the wicked, whom he asks God to slay (Psalm 139:19). David does not vow to kill evil persons himself. Nor does he ask God to make him an executioner. Rather, David brings his heartache to God, and leaves the results in the Lord's hands. David is also fully aware that his own heart is fallible (Psalm 139:23–24).

First, these evil ones speak against God with malicious intent. They purposely insult God. Psalm 73:9 also accuses the wicked of such an offense against God. There Asaph wrote, "They set their mouths against the heavens, and their tongue struts through the earth." Jude 1:15 describes apostates as ungodly sinners that have spoken harsh things against the Lord. Jude calls them "grumblers, malcontents, following their own sinful desires" (Jude 1:16). He also calls them "loud-mouthed boasters".

Second, David levels the charge that the wicked take God's name in vain (Psalm 139:20). Only those entirely deaf, it seems, can escape hearing God's name taken in vain on a daily basis. His name is profaned on TV, in the workplace, in movies, in the marketplace, at sports venues—virtually everywhere. Such language, whether intentional or thoughtless, violates the moral principle to "not take the name of the LORD your God in vain" (Exodus 20:7; Luke 6:46; Philippians 2:9–10). The statement of this concept in the ten commandments vows "for the LORD will not hold him guiltless who takes his name in vain" (Exodus 20:7).