...before the morning he is not.

Just because God is punishing Israel for covenant abandonment, it doesn't mean the conquerors are "good guys" in God's eyes.

Isaiah 17:14

14 And behold at eveningtide trouble; and before the morning he is not. This is the portion of them that spoil us, and the lot of them that rob us.

Great things are not happening just because something like Babylon is destroying Judea. It isn't like the Lord got a new friend. They are "not" for spoiling the children of Israel, suffering the consequences for oppressing God's people. 

Jews are being punished for turning from the Lord, who is really being a judge to all, punishing one for their sins and punishing another for their sins. The Lord doesn't change favorites nor does he walk away from covenants - the Jews are the covenant people and their inattention to their promises will be punished.