What happened to Joash at the end of his life?

Answer: Killed by his own servants

Joash's servants formed a conspiracy and struck him down at Beth-millo; his son Amaziah reigned in his place.

“And the servants of Joash rose up and formed a conspiracy and killed him at Beth-millo, on the road down to Silla. His servants Jozabad son of Shimeath and Jehozabad son of Shomer struck him down, and he died. And they buried him with his fathers in the City of David, and his son Amaziah reigned in his place.”— 2 Kings 12:20-21 (BSB)