The execution of John the Baptist, in Mark 6:14-29, is a story that goes beyond mere political corruption. The contrast between the innocence of a young girl, who at the bidding of her mother, asks for the head of John on a platter, and Herod's motives, is grotesque and reveals the potential for utter corruption and evil of those in power.
Yielding to pride and his weakness, Herod executes John because he “vowed” that he would give the young girl whatever she wanted. Of course as king, he was not required by law to fulfill a vow or execute an innocent man at her request.
This story shares many common elements with the story of the trial before Pilate to come. Pilate also condemns Jesus to death knowing that he had done nothing wrong. Jesus' execution is ordered out of political expediency and out of Pilate's desire to save his reputation and his own political future. In the same way, Herod clearly has John executed rather than suffer the political embarrassment of not keeping his vow.