2 Kings 10:15-17. As Jehu is in the middle of getting rid of the house of Ahab and every person loyal to it, he meets Jehonadab, son of Rachab. Jehu asks Jehonadab if his heart is true to Jehu's heart and Jehonadab says it is. So Jehu bringsJehonadab with him on his chariot and together they go to Samaria. There, Jehu "slew all that remained to Ahab in Samaria, till he had wiped them out, according to the word of the Lord which he spoke to Elijah."

All this slaughter in the Old Testament often seems so wrong to us, but God did speak these words to Jehu through Elijah. So there is such a thing as a just war or as just killing. It is very rare, and as the world becomes more civilized it is rarer still. But in Jehu's time, the way to establish justice was often through the death of those who were unjust. Even so, that decision is not for a person to make. Jehu's actions would not have been right or just unless God had told him to do this. But God did tell him to do this, through Elijah.

Ahab and Jezebel's worship of Baal was so harmful and so entrenched that it was hurting them and God's people immeasurably. God, in his Providence, had determined that the only way to save his people was to wipe out Ahab and his entire family, and God chose Jehu to do it. Notice that Jehu is seeking only to do God's will. He stops and asks Jehonadab where his heart lies. Jehu does not want to risk killing anyone that God does not want him to kill. He is a true instrument of God's justice.