And you shall strike down the house of Ahab your master, so that I may avenge on Jezebel the blood of my servants the prophets, and the blood of all the servants of the LORD.
We have here the anointing of Jehu to be king, who was, at this time, a commander (probably commander-in-chief) of the forces employed at Ramoth-Gilead, Kg2 9:14. There he was fighting for the king his master, but received orders from a higher king to fight against him.
Commenting on 2 Kings 9:1-10
For the whole house of Ahab shall perish,.... Be cut off by death, and that in a violent manner, not one should escape: and I will cut off from Ahab him that pisseth against the wall, and him that is shut up and left in Israel; of these phrases; see Gill on Kg1 14:10, Kg1 21:21
Thou shalt smite the house of Ahab - For their most cruel murders they have forfeited their own lives, according to that immutable law, "He that sheddeth man's blood by man shall his blood be shed." This and the two following verses contain the commission which Jehu received from the Lord against the bloody house of Ahab.