Albert Barnes Commentary 1 Kings 20:2

"And he sent messengers to Ahab king of Israel, into the city, and said unto him, Thus saith Ben-hadad," — 1 Kings 20:2 (ASV)

It is likely that a considerable time had passed in the siege, that the city had been reduced to a desperate situation, and that ambassadors had been sent by Ahab to ask for terms of peace short of absolute surrender, before Ben-hadad would make such a demand. He would expect and intend his demand to be rejected, and this would have left him free to plunder the town, which was evidently what he desired and intended.