Does God ever reject mercy?

there is an account in 1 Kings 20 where the king of Aram, Ben Hadad, attacks Israel (Samaria) twice within a year and God promises to deliver them into the hands of Ahab. The second time the Arameans suffered great losses and Ben Hadad and the 32 kings that accompanied him decided to present themselves to king Ahab in sackcloth with ropes around their necks (I assume to mean “we deserve to die”). Ben Hadad’s officials advised him to do this saying that they had heard that the kings of the house of Israel were merciful.

Here’s the shocker that makes us stop and ask questions. King Ahab actually treats Ben Hadad like a brother and drives him around in his chariot while BH promises to return cities and give him commercial opportunities. God delivers and mercy is shown…a great picture right? Wrong! God’s prophet (unnamed) sets up a ruse to get the kings attention (by the way, one of his companions gets killed by a lion for not wounding him with a sword when he asks). He condemns the king and says the words of the Lord, “You have set free a man I had determined should die. Therefore it is your life for his life, your people for his people.”