Psalm 140 models the prayer of the faithful when confronted with the countless stories, and even personal experiences, of violent men acting wickedly.
We pray during times of wickedness, because we are not ignorant and naïve. We know violent men abound (v. 1), that they plan their evil exploits (v. 2, 4), use their words for evil (v. 3) and do it all with an arrogant boastfulness (v. 5).
But not only do we pray to God because of our knowledge of the violent men, but we also pray to God because of our knowledge of Him. God is our God (v. 6) who hears our prayers (v. 6). He has protected us before (v. 7), and can frustrate the plans of the wicked (v. 8) to the point of their own violent destruction (vv. 8-11). We know with certainty that God will bring final and complete justice (v. 12).
When our knowledge of evil men is met with the knowledge of God, then we can truly praise the Lord in the meantime anticipating the perfect end to it all (v. 13).