Joab seems to have had a serious thought, that the motive in the mind of David was not right.
Related Commentaries of 1 Chronicles 21:3
1 Chronicles 21:3-4
3 And Joab answered, The LORD make his people an hundred times so many more as they be: but, my lord the king, are they not all my lord's servants? why then doth my lord require this thing? why will he be a cause of trespass to Israel?
4 Nevertheless the king's word prevailed against Joab. Wherefore Joab departed, and went throughout all Israel, and came to Jerusalem.