John Gill Commentary Psalms 37:8

"Cease from anger, and forsake wrath: Fret not thyself, [it tendeth] only to evil-doing." — Psalms 37:8 (ASV)

Cease from anger
Either at these wicked men who are so prosperous, or at God, who for the present suffers it; see (Jonah 4:9) (Proverbs 19:3) ;

and forsake wrath ;
which is anger wrought up to a greater degree; and the rather to be shunned and avoided, as being very disagreeable to the character of a good man;

fret not thyself in any wise to do evil ;
evil may be done by fretting at the prosperity of wicked men, or by imitating them, doing as they do, in hope of being prosperous as they are; from which the psalmist dissuades by reasons following.