Charles Ellicott Commentary 1 Samuel 12:21

"and turn ye not aside; for [then would ye go] after vain things which cannot profit nor deliver, for they are vain." — 1 Samuel 12:21 (ASV)

For then should you go after vain things. —The passage is more forcible without the “for” and the words in italics supplied in the English translation. The verse without it would run thus: Turn you not aside after vain things which cannot profit, etc. Singularly enough, not one of the ancient versions translates the Hebrew ki, “for”: they all omit it. It is therefore clear that this “for” has, through some copyist’s error, got into the text since the versions were made.