C. F. Keil and Franz Delitzsch on Deuteronomy 13:9-11

Deu 13:9-11

To such persuasion Israel was not to yield, nor were they to spare the tempters. The accumulation of synonyms (pity, spare, conceal) serves to make the passage more emphatic. כּסּה, to cover, i.e., to keep secret, conceal. They were to put him to death without pity, viz., to stone him (cf. Lev 20:2). That the execution even in this case was to be carried out by the regular authorities, is evident from the words, “thy hand shall be first against him to put him to death, and the hand of all the people afterwards,” which presuppose the judicial procedure prescribed in Deu 17:7, that the witnesses were to cast the first stones at the person condemned.