Matthew 14:3

‘For Herod had laid hold on John, and bound him, and put him in prison for the sake of Herodias, his brother Philip's wife.'

We are now told why Herod was upset at the idea of John coming back from the grave. It was because of the way that he himself had treated him. Herod had gone on a visit to see his brother Philip (not the tetrarch Herod Philip) and had fallen in love with Philip's wife, Herodias, who spotting the opportunity of greater prestige and influence had yielded to Herod's entreaties and had divorced her husband and married him. But such behaviour was forbidden by Jewish Law. A man could not marry the wife of his brother while his brother was still alive.