Albert Barnes 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)

A conspiracy - Compare this with the marginal reference. For some unknown reason, possibly a suspicion of treason, Joash took up residence in the fortress of Millo (1 Kings 9:24). This conspiracy was connected to religion. Soon after the death of Jehoiada, Joash had apostatized, renewed the worship of Baal, and persisted in his evil ways despite many prophetic warnings. He even commanded Zechariah to be slain when he rebuked these actions (2 Chronicles 24:18–27).

The conspirators sought to avenge Zechariah and, no doubt, also to put an end to Baal worship. In this, it appears they succeeded. For although Amaziah eventually punished the actual murderers (2 Kings 14:5), he does not seem to have been a Baal-worshipper. The only idolatries attributed to him are maintaining the high places (2 Kings 14:4) and worshipping the gods of Edom (2 Chronicles 25:14–20).

Silla - This place is completely unknown.