II Samuel 24:3

Numbering of the People, and Pestilence - 2 Samuel 24 For the purpose of ascertaining the number of the people, and their fitness for war, David ordered Joab, his commander-in-chief, to take a census of Israel and Judah. Joab dissuaded him from such a step; but inasmuch as the king paid no attention to his dissuasion, he carried out the command with the help of the military captains (Sa2 24:1-9). David very speedily saw, however, that he had sinned; whereupon the prophet Gad went to him by the command of Jehovah to announce the coming punishment, and give him the choice of three different judgments which he placed before him (Sa2 24:10-13). As David chose rather to fall into the hand of the Lord than into the hand of men, God sent a pestilence, which carried off seventy thousand men in one day throughout the whole land, and had reached Jerusalem, when the Lord stopped the destroying angel in consequence of the penitential prayer of David (Sa2 24:14-17), and sent Gad to the king to direct him to build an altar to the Lord on the spot where the destroying angel had appeared to him (Sa2 24:18). Accordingly David bought the threshing-floor of Araunah the Jebusite, built an altar upon it, and sacrificed burnt-offerings and thank-offerings, after which the plague was stayed (Sa2 24:19-25). This occurrence, which is introduced in the parallel history in 1 Chron 21 between David's wars and his arrangements for a more complete organization of the affairs of the nation, belongs undoubtedly to the closing years of David's reign. The mere taking of a census, as a measure that would facilitate the general organization of the kingdom, could not in itself be a sinful act, by which David brought guilt upon himself, or upon the nation, before God. Nevertheless it is not only represented…

24:3 why . . . do you want to do this? Joab either realized that such a move would be unpopular among the people, or he believed that it was wrong before God.