Jehu assembled the followers of Baal and then slaughtered them all

After Jehu finished killing Ahab's family in Samaria, he called an assembly to worship Baal.

Jehu gathered all the people together, and said unto them, Ahab served Baal a little; but Jehu shall serve him much. 2 Kings 10:18

Jehu would lead the sacrifice to Baal and any follower of Baal that did not come would be killed.

Now therefore call unto me all the prophets of Baal, all his servants, and all his priests; let none be wanting: for I have a great sacrifice to do to Baal; whosoever shall be wanting, he shall not live. 2 Kings 10:19a

But he was lying, of course. He assembled the followers of Baal, not to worship Baal with them as he said, but to kill them all.

But Jehu did it in subtilty ... that he might destroy the worshippers of Baal. 2 Kings 10:19b

So Jehu called the assembly.

Jehu said, Proclaim a solemn assembly for Baal. And they proclaimed it. And Jehu sent through all Israel: and all the worshippers of Baal came, so that there was not a man left that came not. And they came into the house of Baal; and the house of Baal was full from one end to another. 2 Kings 10:20-21

Then he told 80 of his men to kill of the worshipers of Baal, saying he'd kill whoever let any escape.

And when they went in to offer sacrifices and burnt offerings, Jehu appointed fourscore men without, and said, If any of the men whom I have brought into your hands escape, he that letteth him go, his life shall be for the life of him. 2 Kings 10:24

When Jehu finished his sacrifice to Baal, he ordered the sacrifice of all of the followers of Baal.

As soon as he had made an end of offering the burnt offering, that Jehu said to the guard and to the captains, Go in, and slay them; let none come forth. And they smote them with the edge of the sword. 2 Kings 10:25

God approved of this and all of Jehu's previous mass murders. He liked them so much, in fact, that Jehu's sons would be kings of Israel for the next four generations.

And the LORD said unto Jehu, Because thou hast done well in executing that which is right in mine eyes, and hast done unto the house of Ahab according to all that was in mine heart, thy children of the fourth generation shall sit on the throne of Israel. 2 Kings 10:25

God's next killing: The priest of Baal and Queen Athaliah