The pride of the Assyrian king is repugnant to the Lord. He will not allow him to destroy the kingdom of Judah. Instead, he promises Assyrian destruction. This comes because the Assyrian king had taken all the credit for his military successes, 'For he saith: by the strength of my hand and by my wisdom I have done these things; for I am prudent.'
This pride is similar to the pride of the wicked Nephites in the last battles with the Lamanites:
'And in the three hundred and sixty and second year they did come down again to battle. And we did beat them again, and did slay a great number of them, and their dead were cast into the sea.
And now, because of this great thing which my people, the Nephites, had done, they began to boast in their own strength, and began to swear before the heavens that they would avenge themselves of the blood of their brethren who had been slain by their enemies.
And they did swear by the heavens, and also by the throne of God, that they would go up to battle against their enemies, and would cut them off from the face of the land.
And it came to pass that I, Mormon, did utterly refuse from this time forth to be a commander and a leader of this people, because of their wickedness and abomination.' (Mormon 3:8-11)