Cease from anger, and forsake wrath; i.e. such anger and such wrath as the prosperity of the wicked calls forth. Fret not thyself in any wise to do evil; rather, fret not thyself, only to do evil. No result could be looked for from the sort of "fretting" spoken of, but an evil one.
If men will dwell unduly on the fact of the prosperity of the wicked, and brood upon it in their hearts, they will be apt, in the first instance, to envy the wicked, which is at once "to do evil;" and from this they will be naturally tempted to go on to an imitation of their wicked practices, which is to assimilate themselves altogether to the enemies of God, and to be guilty of practical apostasy (comp.
Psalms 73:2, "But as for me, my feet were almost gone; my steps had well-nigh supped. For I was envious at the foolish, when I saw the prosperity of the wicked").