For when David was up - It is supposed that David's contrition arose from the reproof given by Gad, and that in the order of time the reproof came before the confession stated in the 10th verse.
David's seer - A holy man of God, under the Divine influence, whom David had as a domestic chaplain.

Go, and say unto David,.... Not my servant David, as Nathan was bid to say to him when it was in his heart to build an house for him, Sa2 7:5; but now he had sinned and displeased the Lord, and therefore it is only plain David:

thus saith the Lord, I offer thee three things; or lay them before thee to consider of which thou wouldest have done; the Targum is,"one of three things I cast upon thee,''as a burden to bear; one of the three I will certainly inflict upon thee by way of chastisement:

choose thee one of them, that I may do it unto thee; here is mercy mixed with judgment; the Lord is angry, yet shows great condescension and goodness; a sovereign Being, who could have imposed what punishment he pleased, and even all the three after mentioned, yet resolves but on one, and leaves that to the option of David.

HE, HAVING THREE PLAGUES PROPOUNDED BY GAD, REPENTS, AND CHOOSES THREE DAYS' PESTILENCE. (Sa2 24:10-14)

David's heart smote him after that he had numbered the people. And David said unto the Lord, I have sinned--The act of numbering the people was not in itself sinful; for Moses did it by the express authority of God. But David acted not only independently of such order or sanction, but from motives unworthy of the delegated king of Israel; from pride and vainglory; from self-confidence and distrust of God; and, above all, from ambitious designs of conquest, in furtherance of which he was determined to force the people into military service, and to ascertain whether he could muster an army sufficient for the magnitude of the enterprises he contemplated. It was a breach of the constitution, an infringement of the liberties of the people, and opposed to that divine policy which required that Israel should continue a separate people. His eyes were not opened to the heinousness of his sin till God had spoken unto him by His commissioned prophet.