The following is from "Jesus: A Revolutionary Biography", by John Dominic Crossan, pages 35 - 36.
Antipas's execution of John cannot be explained by a simple appeal to Mark 6:17-29, even if one took that marvelous fiction as historical fact. Mark's account is best seen as his own creation, allowing him to emphasize certain parallels between the fate of John and Jesus, especially how both were put to death at the insistence of others by a reluctant and almost guiltless civil authority — Antipas for one, Pilate for the other.
In life, death, and even burial by disciples, John is, for Mark, the precursor of Jesus. And, probably, he was deliberately recalling an earlier and well-known Mediterranean horror story. When, in 184 b.c.e., Cato was one of the two official censors at Rome, he had Lucius Quinctius Flaminius expelled from the senate despite his consular rank. His crime is described by the orator Cicero, who died in 43 b.c.e.; again by the historian Livy, who died in 17 c.e.; and finally by the rhetorician Seneca the Elder, who died in 40 c.e. Here is one of the two versions in Livy's history of Rome, Book 3943:3-4.
At Placentia a notorious woman, with whom Flaminius was desperately in love, had been invited to dinner. There he was boasting to the courtesan, among other things, about his severity in the prosecution of cases and how many persons he had in chains, under sentence, whom he intended to behead. Then the woman, reclining below him, said that she had never seen a person beheaded and was very anxious to behold the sight. Hereupon, he says, the generous lover, ordering one of the wretches to be brought to him, cut off his head with his sword. This deed ... was savage and cruel: in the midst of drinking and feasting, where it was the custom to pour libations to the gods and to pray for blessings, as a spectacle for a shameless harlot, reclining in the bosom of a consul, a human victim sacrificed and bespattering the table with his blood!
The point was not that the man was innocent; he was going to be executed in any case. But it should still not be done just to please a mistress, and not at a banquet. The story was clearly a well-known example of how not to exercise power. Mark's creation intends, most likely, to recall that classic model.