2 Kings 9:7

thou shalt smite the house of Ahab thy master The punishment of Ahab had been postponed because of his repentance (1 Kings 21:29) but the sins of the father, which have blossomed and borne evil fruit in his children, will at last be thoroughly punished. The instrument chosen is one who had been in the service of Ahab, and so was aware of the evil that had been wrought by him.

my servants, the prophets Although Obadiah was able to save a hundred, Jezebel still made havoc of the rest. And it appears from the words that follow that not the prophets only, but all the servants of the Lord, had been the objects of her rage. And she was yet alive and in power. There must have been much zeal for Jehovah to sustain the righteous spirit which manifests itself so frequently in the history of Elijah and Elisha.

at the hand of Jezebel i.e. God will require from her and her family the penalty for all the evil that has been wrought.