Adam Clarke on Matthew 14:9

The king was sorry - He knew John to be a righteous man, and at first did many things gladly which John told him it was his duty to perform: Mar 6:20.

Nevertheless, for the oath's sake - The Oaths, ορκους - he had probably sworn again and again - one sin begets many.

And them which sat with him at meat - Who were probably such as himself, and would have considered it a breach of honor if he had not fulfilled his sworn promise: he therefore commanded it to be given!