Albert Barnes Commentary Psalms 58:7

"Let them melt away as water that runneth apace: When he aimeth his arrows, let them be as though they were cut off." — Psalms 58:7 (ASV)

Let them melt away as waters which run continually - Let them vanish or disappear as waters that flow off, or floods that run by, and are no more seen. Perhaps the allusion here may be to the waters of a torrent that is swollen, which flow off and are lost in the sand, so that they wholly disappear. See the notes at (Job 6:15–19). The prayer is, that his enemies might perish or be cut off, and that he might thus be saved from them.

When he bendeth his bow to shoot his arrows - literally, “he treads on his arrows.” See the notes at (Psalms 11:2). The meaning here is, when he prepares for an attack - or, prepares to make war, as one does who bends his bow and places his arrow on the string. The allusion here is to the enemies of David, as seeking his life.

Let them be as cut in pieces - That is, let his arrows be as if they were cut off or “blunted,” so that they will produce no effect. Let them be such that they will not penetrate and wound.