Albert Barnes on Matthew 14:3-5

For Herod had laid hold on John, etc. See Mr 6:17-20 Lu 3:19,20. This Herodias was a grand-daughter of Herod the Great. She was first married to Herod Philip, by whom she had a daughter, Salome, probably the one that danced and pleased Herod. Josephus says that this marriage of Herod Antipas with Herodias took place while he was on a journey to Rome. He stopped at his brother's; fell in love with his wife; agreed to put away his own wife, the daughter of Arteas, king of Petraea; and Herodias agreed to leave her own husband, and live with him. They were living, therefore, in adultery; and John in faithfulness, though at the risk of his life, had reproved them for their crimes. Herod was guilty of two crimes in this act:

(1) of adultery, as she was the wife of another man;

(2) of incest, as she was a near relation, and such marriages were expressly forbidden, Lev 18:16.