John Gill Bible Commentary

Cursed be he that doeth the work of the Lord deceitfully,.... Which is said with respect to the Chaldeans, who were enjoined to destroy the Moabites; which is called the work of the Lord, because he had given them a commission to do it; and which was to be done by them, not by halves, or in a remiss and negligent manner, but fully and faithfully; they were not to spare them, as Saul did the Amalekites, and Ahab Benhadad. This is a general rule, which may be applied to all divine work and service; every man has work to do for God; some in a more public, others in a more private way; all should be done in uprightness and sincerity, with all faithfulness and integrity: it is done deceitfully when men play the hypocrite; and negligently when they are backward to it, lukewarm in it, and infrequent in the performance of it; which brings upon them the curse of God; and which is not a curse causeless, but a legal one; and is no other than the wrath of God in strict justice: and cursed be he that keepeth back his sword from blood; from shedding the blood of the Moabites, when God had given command to do it. The curse is repeated, as Kimchi observes, to confirm the matter, that it might be most assuredly expected; since it would certainly come, if the Lord's work was not done aright.