The development of modern Biblical studies has taught us not to be surprised at the differences in the Gospel accounts of any event or in the way in which the different writers emphasize different aspects of a particular person's character.
Thus the St. John's Gospel concentrates on John the Baptist's witness to Jesus and gives more importance to this ministry of witnessing than to his ministry of baptizing. There is no contradiction here, of course, and in fact the two ministries come together in several different ways. Both ministries prepared for the coming of Jesus and so brought the life of John into close relationship with Jesus, a reality which we first see in the Angel's annunciation to Mary and then in Mary's visit to Elizabeth.
Herod, of course, did not know enough to associate John and Jesus and historically speaking, we may say that John's death was not related to the ministry of Jesus. John was not arrested because of his baptizing ministry but because he preached repentance and the truth, two important points in Jesus' ministry of evangelization. Finally John's death was his greatest act of witness, witness to God's truth and call to repentance, which were embodied more fully in the life and death of Jesus.
Lord Jesus, may our lives always bear witness to Your liberating truth.