At Solomon's command, who struck down Joab at the altar?

Answer: Benaiah son of Jehoiada

Joab fled to the tent of the LORD and held the horns of the altar, but Benaiah son of Jehoiada went in and killed him.

“It was reported to King Solomon: “Joab has fled to the tent of the LORD and is now beside the altar.” So Solomon sent Benaiah son of Jehoiada, saying, “Go, strike him down!””— 1 Kings 2:29 (BSB)