John Gill Commentary 2 Kings 12:20

"And his servants arose, and made a conspiracy, and smote Joash at the house of Millo, [on the way] that goeth down to Silla." — 2 Kings 12:20 (ASV)

And his servants arose
This was after he had slain Zechariah the son of Jehoiada, who reproved him for his idolatry; and after a second expedition of the king of Syria, who came to Jerusalem, and spoiled it, and left Jehoash diseased, as is recorded in (2 Chronicles 24:23–26) ,

and made a conspiracy ;
not to get the kingdom into their hands, for his son succeeded him, but to avenge the death of Zechariah;

and slew Jehoash in the house of Millo, which goeth down to Silla ;
these are both names of places; perhaps the latter is mentioned, to distinguish this Bethmillo from Millo in Zion; or rather that itself is meant, and described by the descent from it to a causeway, as Silla may signify, which led to the royal palace.