Albert Barnes Commentary 2 Kings 21:2

"And he did that which was evil in the sight of Jehovah, after the abominations of the nations whom Jehovah cast out before the children of Israel." — 2 Kings 21:2 (ASV)

During his youth, Manasseh naturally fell under the influence of the leading Jewish nobles, with whom the pure religion of Yahweh was always unpopular (Compare to 2 Chronicles 24:17–18; Jeremiah 8:1–2). They seem to have persuaded him not only to undo Hezekiah’s work but also to go to unprecedented lengths in polytheism, magic, and idolatry.

The sins of Manasseh’s reign appear to have been what filled up the measure of Judah’s iniquity and brought down the final sentence of doom on the last remnant of the chosen people (2 Kings 23:26).