Hath justified herself - Judah had had the benefit of the warning given by Israel’s example. Both abandon Yahweh’s service for idolatry, but Israel is simply “apostate,” Judah is also false.
The verse is important,
(1) as accounting for the destruction of Jerusalem so soon after the pious reign of Josiah. Manasseh’s crimes had defiled the land, but it was by rejecting the reforms of Josiah that the people finally profaned it, and sealed their doom:
(2) As showing that it is not by the acts of its government that a nation stands or falls. Ahaz and Manasseh lent the weight of their influence to the cause of idolatry: Hezekiah and Josiah to the cause of truth. But the nation had to determine which should prevail. Excepting a remnant it embraced idolatry, and brought upon itself ruin: in the remnant the nation again revived Jeremiah 24:5, Jeremiah 24:7.