Virtually everything in Mark 6 is found in the other gospel accounts, but Mark gives significantly more attention to the death of John the baptist.
The Herod of this story is Herod Antipas, the son of Herod the Great (who killed the little babies when Jesus was born). Antipas is the Herod of Judea during the story of Jesus. He had a half-brother, Aristobulus, and and two other half-brothers, both named Philip. One was Herod Philip, the governor of Iturea, the other was not a ruler, but simply known as “Philip.” Herodias was Aristobulus’s daughter. She married her uncle, Philip (the one who was not a ruler). The two of them had a child, Salome.
On a trip to Rome, Herod Antipas, already married for over twenty years to the daughter of the king of Nabatea, visited with Philip, met Herodias, his niece, and fell in love with her. He asked her to marry him, and she agreed, provided he leave his present wife. He agreed. The Old Testament forbade marrying your brother’s wife (Leviticus 20:21). On this basis, John condemned Herod Antipas. His preaching didn’t bother Herod much, but it did bother Herodias. At a gathering of some important dignitaries, Herodias’ daughter danced and so impassioned her step-father that he offered her half his kingdom. At the suggestion of Herodias, she asked for the head of John the baptist. It’s a repulsive story.
The chapter is full of hard-hearted people. There are those in Jesus’ home town who, knowing his reputation as a miracle-worker would none-the-less not bring their sick to him. Out of spite they would snub the home town boy made good. There is Antipas, whose passion caused him to murder a prophet. There is Herodias, whose quest for power caused her to disobey the law.
And then, at the end of the chapter, there are the disciples. Though Jesus can still a storm with a rebuke, they cannot believe who he is because, their hearts were hard.
Hard-heartedness blinds us to the truth, draws us away from the will of God, enslaves us to sin, and keeps us from enjoying the blessings God intended for His people.