John Gill Commentary Mark 6:18

"For John said unto Herod, It is not lawful for thee to have thy brother`s wife." — Mark 6:18 (ASV)

For John had said to Herod
He had been personally with him, and had plainly and freely told him of his sin, and faithfully reproved him for it; alleging what he had done was contrary to the law of God: it is not lawful, for you to have your brother's wife : (See Gill on Matthew 14:4).