Who conspired against and killed King Pekah of Israel?

Answer: Hoshea

Hoshea son of Elah conspired against Pekah and assassinated him.

“Then Hoshea son of Elah led a conspiracy against Pekah son of Remaliah. In the twentieth year of Jotham son of Uzziah, Hoshea attacked Pekah, killed him, and reigned in his place.”— 2 Kings 15:30 (BSB)