God's decreed word
God had decreed that they will be taken to Babylon and those who were repentant will survive by submitting to the Babylonian king and in later years will return back
Jeremiah 29:10 NASB
10 “For this is what the Lord says: ‘When seventy years have been completed for Babylon, I will visit you and fulfill My good word to you, to bring you back to this place.
10 “For this is what the Lord says: ‘When seventy years have been completed for Babylon, I will visit you and fulfil my good word to you, to bring you back to this place.
God's integrity
So by escaping to Egypt they were thumbing their nose at God's judgement and subverting the Babylonian captivity of seventy years. God had also commanded them (Deuteronomy 17:16) never to return to Egypt which was coincidental a place were he had redeemed them from.
God's punishment
Jeremiah 44:28-29 NASB
28 Those who escape the sword will return from the land of Egypt to the land of Judah [ac]few in number. Then all the remnant of Judah who have gone to the land of Egypt to reside there will know whose word will stand, Mine or theirs. 29 And this will be the sign to you,’ declares the Lord, ‘that I am going to punish you in this place, so that you may know that My words will assuredly stand against you
28 Those who escape the sword will return from the land of Egypt to the land of Judah [ac]few in number. Then all the remnant of Judah who have gone to the land of Egypt to reside there will know whose word will stand, Mine or theirs. 29 And this will be the sign to you,’ declares the Lord, ‘that I am going to punish you in this place, so that you may know that My words will assuredly stand against you
So Jeremiah' strong opposition to Jews settling in Egypt was based on God's word and integrity that was at stake
