Charles Ellicott Commentary 1 Chronicles 21:3

"And Joab said, Jehovah make his people a hundred times as many as they are: but, my lord the king, are they not all my lord`s servants? why doth my lord require this thing? why will he be a cause of guilt unto Israel?" — 1 Chronicles 21:3 (ASV)

Answered. —Hebrew, said.

The Lord ... as they be. —Literally, Jehovah add upon his people like them a hundred times, an abridged form of what is read in Samuel.

But, my lord the king, are they not ...? —Instead of this, Samuel records another wish, And may the eyes of my lord the king be seeing, that is, living (Genesis 16:13).

Why then does my lord require this thing? —So Samuel, in slightly different terms: And my lord the king, why desireth he this proposal?

Why will he be (why should he become) a cause of trespass to Israel? —Not in Samuel. It is an explanatory addition by the chronicler.