Bible Notes Online - 2-Kings 21 - ESV
Commentary

v1-2: The reign of Manasseh fell in the gap between the early prophets, such as Isaiah, Hosea, Amos, and the later prophets, Jeremiah, Zephaniah, Ezekiel.

v3-6: Manasseh's deliberate choice to sin, and to promote sin; he copied the Gentile nations with their false and empty religion. He was committed to sin, since he did "much evil". He introduced idolatry into the temple of the Lord, while other kings had 'added' idolatry to true worship; Manasseh sought to replace true worship with idolatry, he put idols in the places set apart for the Lord's things.

Manasseh followed the practce of child sacrifice, part of the worship of the idol, Molech. See notes on 1 Kings 11. Manasseh filled Jerusalem with innocent blood, the blood of children. 

v7-9: Manasseh and the people ignored the clear instructions found in the word of God. They actually did more evil that the Canaanites nations which had been destroyed.

We dare not think of Canaanites as uncivilised. Their literature was impressive. Their linear alphabet became the basis for the modern western alphabet. These people were not uncivilised savages. They were civilised idolaters. We do not equate idolatry with lack of intelligence.

v10-15: The prophetic message, from anonymous prophets; because of Manasseh's sins, Judah would be judged and defeated. Manasseh made this judgment inevitable and unavoidable. Josiah's godly reign could only delay its fulfilment.

v16-18: Manasseh was not buried with his fathers; see 15.38; 16.20, and v38. Manasseh stood out from the other kings of Judah; he excelled in sins, and adversely affected Judah for many years. He was remembered for the sins he committed.

v19-26: Amon's short reign merits just a few verses. He continued the pattern of his father's reign. He was killed by his officials, who in turn were put to death by the people of the land.