A.T. Robertson Commentary Mark 6:17

"For Herod himself had sent forth and laid hold upon John, and bound him in prison for the sake of Herodias, his brother Philip`s wife; for he had married her." — Mark 6:17 (ASV)

For Herod himself (Αυτος γαρ ο Hηρωιδης). Mark now proceeds to give the narrative of the death of John the Baptist some while before these nervous fears of Herod. But this post eventum narrative is very little out of the chronological order. The news of John's death at Machaerus may even have come at the close of the Galilean tour. "The tidings of the murder of the Baptist seem to have brought the recent circuit to an end" (Swete). The disciples of John "went and told Jesus. Now when Jesus heard it, he withdrew from thence in a boat" (Matthew 14:12f.). See on Mt 14:3-12 for the discussion about Herod Antipas and John and Herodias.