Jehu was on a mission from God.
(Like Anton Chigurh in "No Country for Old Men," but without the coin tosses.)
First he killed one of Ahab's sons, Jehoram. Then he killed Ahab's wife, Jezebel. Now it was time to take care of the rest of Ahab's family.
He started by writing letters to the rulers of Jezreel challenging them fight for Ahab's sons. But they were intimidated by Jehu (they'd probably heard about his previous murders) and said that they would do whatever he wanted.
So Jehu wrote another letter telling them what he wanted: Bring him the heads of all 70 of Ahab's sons tomorrow.
Then he wrote a letter the second time to them, saying, If ye be mine, and if ye will hearken unto my voice, take ye the heads of the men your master's sons, and come to me to Jezreel by to morrow this time. Now the king's sons, being seventy persons, were with the great men of the city, which brought them up. 2 Kings 10:6And that's what they did. They killed Ahab's 70 sons, put the heads in baskets, and brought them to Jehu.
And it came to pass, when the letter came to them, that they took the king's sons, and slew seventy persons, and put their heads in baskets, and sent him them to Jezreel. 2 Kings 10:7When Jehu heard that the heads had arrived, he said to put them in two heaps at the city gate and let them stay there overnight.
And there came a messenger, and told him, saying, They have brought the heads of the king's sons. And he said, Lay ye them in two heaps at the entering in of the gate until the morning. 2 Kings 10:8
The next morning Jehu went out and told the people that "the LORD hath done that which he spake by his servant Elijah." (See 1 Kings 21:21)
But there were still others that God wanted Jehu to visit.
God' next killing: Ahab’s hometown family, friends, and priests