John Gill 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)

And he did that which was evil in the sight of the Lord
Was guilty of idolatry:

after the abomination of the Heathen, whom the Lord cast out
before the children of Israel:
the old Canaanites; he committed idolatry in imitation of them, and as the Phoenicians now did before the children of Israel.