Herodias had been married to Herod's brother Herod II, and they had a daughter, Salome. Herod II divorced Herodias. She then married Herod II's brother, Herod Antipas. Herod Antipas had divorced his first wife Phasaelis, the daughter of the Nabatean king, in favor of Herodias.
Into this story of domestic affairs and international intrigue stepped John the Baptist, who condemned the whole sorry mess.
Apparently, Herodias seethed, but her husband, Herod Antipas both feared executing John, and enjoyed his preaching.
For John said to Herod, "It is not lawful for you to have your brother's wife." And Herodias had a grudge against him, and wanted to kill him. But she could not, for Herod feared John, knowing that he was a righteous and holy man, and kept him safe. When he heard him, he was much perplexed; and yet he heard him gladly. (Mark 6:18-20)
We find a similar case in the life of St. Corbinian, the first bishop of Freising (Bavaria), and notable bear-tamer. The local warlord Grimoald was married to his brother's widow, Biltrudis. When St. Corbinian condemned the union, it was Biltrudis whose hatred knew no bounds, and who tried to have the saint killed.
You are no doubt familiar with the story leading up to John's beheading. Herod Antipas' niece, Salome, about whom we know next to nothing, performed a dance for her uncle/step-father after obtaining an extravagant promise of reward. When, prompted by her mother, Herodias, Salome asked for the head of John the Baptist on a platter, Herod lacked the backbone to refuse.
The martyrdom of John the Baptist has been unfortunately eroticized by Victorian museum cheesecake. A 17th Century painting by Carlo Dolci captures the pathos of Salome as pitiful pawn, instead of a half-naked, leering Lolita.
Caravaggio captures the emotionally-charged scene. The disgusted executioner places John the Baptist's head on the platter, held by Salome. She cannot bring herself to look. Is that Herod gazing down at John's head?