N Matthew 12:36-37 we read these words, "Every idle word that men shall speak, they shall give account thereof in the day of judgment. By your words you will be justified or condemned." That's a standard that should make us all quake a little in trepidation and pause. Our tongues are certainly unruly and wild.
However, in this psalm David juxtaposes his words against the words of ungodly men; of unbelievers. In this case, David's words comprised prayers of supplication. Contrariwise, the words of David's enemies were bitter words... harsh and hard to handle. These men had insurrection on their minds and death on their lips. They had chosen as their targets: other men who were better than they were. These evil men were drawn toward each other. With reciprocating hype, they bolstered one another up in their evil conniving. They imagined that they would never be caught. But through their bitter words, the most damage that they did was to themselves (Psalm 64:8).
Let unbelieving men say what they will... the last word will not be theirs (Psalm 64:10). We shouldn't really be surprised when those who are antagonistic against God say things that hurt those of us who have attached ourselves to Him. What we should focus on instead is our own words (Colossians 4:6).