According to 1 Kings 20, why did God kill 100,000 Syrians?

For calling him a god of the hills

And there came a man of God, and spake unto the king of Israel, and said, Thus saith the LORD, Because the Syrians have said, The LORD is God of the hills, but he is not God of the valleys, therefore will I deliver all this great multitude into thine hand, and ye shall know that I am the LORD. ...

And the children of Israel slew of the Syrians an hundred thousand footmen in one day. 1 Kings 20:28-29