Who accused John the Baptist?

Answer 1

John the Baptist openly and publicly condemned Antipas' marriage in 34 CE to his own brother's wife, who divorced her husband in order to marry Herod Antipas. The relevant text from Book 18 of Antiquities of the Jews, by the Jewish historian, Josephus:
"Now many people came in crowds to him, for they were greatly moved by his words. Herod, who feared that the great influence John had over the masses might put them into his power and enable him to raise a rebellion (for they seemed ready to do anything he should advise), thought it best to put him to death. In this way, he might prevent any mischief John might cause, and not bring himself into difficulties by sparing a man who might make him repent of it when it would be too late. Accordingly John was sent as a prisoner, out of Herod's suspicious temper, to Macherus, the castle I already mentioned, and was put to death."

In effect, John accused himself, and subsequently paid the price by his execution.

Answer 2

Herod Antipas had John arrested for condemning his relationship with Herodias. Interestingly, Herodias felt more threatened by him than her husband did, & sought to have him executed.