The psalmist is asking the Lord to do something about liars, people who are proponents of error and rebellion against Him. He has done so in the law of SowingAndReaping and He will take final action in TheJudgment. But, in the present world, it is up to good men to displace evil men by whatever means is necessary.
David was a man of war. He knew full well what it took to displace the wicked.
The law of SowingAndReaping hunts the violent down and will overthrow them.
Violence is typically associated with physical force but violence is anything that comes against light and Truth to kill, steal or destroy.