John Gill Commentary Psalms 18:40

"Thou hast also made mine enemies turn their backs unto me, That I might cut off them that hate me." — Psalms 18:40 (ASV)

You have also given me the necks of my enemies
Either to slay them, or to trample or put a yoke upon them; or rather the sense is, you have made them to fly before me, to turn their necks or backs to me, as the word is used in (Joshua 7:8) ; and it signifies an utter rout and vanquishing of them;

that I might destroy them that hate me ;
as they were unable to face him and stand against him.