Let’s review some of the details of this chapter. Jehu called for the execution of Ahab's 70 surviving sons. They WERE killed and their heads were sent to Jehu in baskets. He had them piled in 2 piles as striking monuments to the fulfilled prophecies of Elijah (II Kings 10:10). Elijah had proclaimed that Ahab's descendants would be wiped out. Yet, there were many from the family of Ahab who were yet to be eradicated. Jehu successfully went after them all (II Kings 10:11).
After Jehu had dealt with those in Israel who were descendants of Ahab, he "happened" to run into 42 of Ahaziah's brothers (male relatives). Remember, Ahaziah had married into Ahab's family. So, Jehu had them killed too. Then he went into Samaria to deal with the problem of Baal worship. Every priest of Baal came to the temple of Baal (II Kings 10:21), and, after a mock sacrifice, Jehu and 80 of his men killed every last one of Baal's prophets and even destroyed the temple itself with the idols of Baal too.