God's word accomplished what it set out to do, stop Nineveh's wickedness. If Nineveh failed to repent, God would have overthrown them in forty days as he stated. Accomplished. If Nineveh repented, God would show them mercy and not overthrow them. Accomplished.
It was up to Nineveh...
Here is a look into the mind of God.
Jeremiah 18
7 At what instant I shall speak concerning a nation, and concerning a kingdom, to pluck up, and to pull down, and to destroy it;
8 If that nation, against whom I have pronounced, turn from their evil, I will repent of the evil that I thought to do unto them.
9 And at what instant I shall speak concerning a nation, and concerning a kingdom, to build and to plant it;
10 If it do evil in my sight, that it obey not my voice, then I will repent of the good, wherewith I said I would benefit them.
Man has a choice. Nations have choices when faced with the word of God. If you do this, then I will do this.