Psalm 58:4 (b) This serpent is used to describe a wicked person who injures the souls and damages the lives of' others by that which comes out of his mouth.
That which such a person says poisons the hearts and the minds of those who hear. By this means the listeners are deceived by Satan and are led astray from GOD's path. False teachers who invent false religions and preach false doctrines are "adders" (See also Psalm 91:13, the open enemy and the secret foe).