It's a wonder people were still competing for the throne.
Jehu sends letters to the elders and caretakers of Ahab's 70 sons, announcing his plan to come and fight them one by one. The caretakers are afraid because he's already killed two kings. They decide to surrender and write back to that effect. Jehu writes back that the condition of surrender is that they have to bring him the heads of Ahab's sons. So they do, and send their heads in baskets.
When his presents arrive, Jehu instructs his servant to make two piles of heads. In the morning, he shows his people the heads and points out that you can't trust anybody. Then he announces that from now on, they're going to live a righteous life now that they've fulfilled god's instructions to Elisha. Then he kills all the rest of Ahab's family, friends, priests, dogwalkers and ice-cream truck delivery drivers. Then he goes home to Samaria.
On his way, he stops in a shearing house, where some of Ahazia's relatives are working. He asks who they are and they introduce themselves. Then he instructs his servants to kill all 42 of them. For those of you keeping count, that's 70 severed heads and 42 slain sheep shearers in just 14 verses.
As he's leaving, he meets someone called Jehonadab and asks him Is thine heart right, as my heart is with thy heart? (v. 15) Jehonadab says it is and Jehu invites him into the chariot so he can see his zeal for the LORD (v. 16) What does that zeal consist of? Killing more members of the house of Ahab.
Then he goes to his people and says he's now a Baal worshipper. He tells them to gather all the other followers together so they can be pagans together, but really his intent is to kill some more. He gathers them all together in a temple, then instructs his servants to give them all clothes. He then pretends to make a burnt sacrifice, and orders his guards to slaughter all the people in the temple, on pain of death should any escape. Then they tear down the temple and turn it into a toilet. For some reason, he keeps the golden calves which turned people away in the first place.
For all his effort, god promises four generations of rule by his sons. However, Jehu is no better than his predecessors at keeping to god's commandments, so god starts taking away their territory, allowing Hazeal to attack them from all sides. Jehu rules for 28 years.