Zimri, one of his officials, who had command of half his chariots, plotted against him. Elah was in Tirzah at the time, getting drunk in the home of Arza, the palace administrator at Tirzah.
References for 1 Kings 16:9
Zimri came in, struck him down and killed him in the twenty-seventh year of Asa king of Judah. Then he succeeded him as king.
References for 1 Kings 16:10
As soon as he began to reign and was seated on the throne, he killed off Baasha’s whole family. He did not spare a single male, whether relative or friend.
References for 1 Kings 16:11
So Zimri destroyed the whole family of Baasha, in accordance with the word of the LORD spoken against Baasha through the prophet Jehu—
because of all the sins Baasha and his son Elah had committed and had caused Israel to commit, so that they aroused the anger of the LORD, the God of Israel, by their worthless idols.
References for 1 Kings 16:13
As for the other events of Elah’s reign, and all he did, are they not written in the book of the annals of the kings of Israel?
In the twenty-seventh year of Asa king of Judah, Zimri reigned in Tirzah seven days. The army was encamped near Gibbethon, a Philistine town.
References for 1 Kings 16:15
When the Israelites in the camp heard that Zimri had plotted against the king and murdered him, they proclaimed Omri, the commander of the army, king over Israel that very day there in the camp.
Then Omri and all the Israelites with him withdrew from Gibbethon and laid siege to Tirzah.
When Zimri saw that the city was taken, he went into the citadel of the royal palace and set the palace on fire around him. So he died,
because of the sins he had committed, doing evil in the eyes of the LORD and following the ways of Jeroboam and committing the same sin Jeroboam had caused Israel to commit.