What is God doing in the beheading of John the Baptist?

Yesterday’s lectionary gospel reading, Mark 6.14–29, felt distinctly odd by any measure. If you are a good Anglican, and ensure you read not only from the NT and the Psalms but also from the Old Testament every week, it will have been less of a surprise. By contrast, if you are into popular culture and have become a fan of the TV series Game of Thrones, or perhaps play the latest generation of video games, then it will not have seemed so strange. But what is the passage doing here, as part of the ‘good news’ that Mark offers us of Jesus, and why does he give so much time to it in his shortest of gospels—much more time than he gives to his description of the resurrection, the efforts of later editors of the end of the gospels notwithstanding?