This is a sad narrative.  King Herod was trapped by his pride, and afraid of his wife's anger; he agreed to murder a godly man.  The king had married Herodias who was his brother's wife, while his brother was still alive.  It was a wilful act of defiance against God's law (Leviticus 20:21).  Although Herodias was thrilled to have the power of being the 'queen', John the Baptist told Herod that the relationship was not right before God.  That inflamed Herodias.  Putting John in prison protected him from the queen’s anger and also gave Herod the opportunity to continue their interesting discussions (Mark 6:19-20).  

The day came when the king had his birthday party.  We do not know the age of Herodias' daughter, but she was old enough to dance in a sensual way, and young enough to need her mother's opinion.  When her dancing delighted Herod, he made a foolish oath to give her anything she wanted.  The girl went straight to Herodias, who saw her opportunity to get rid of the godly prophet who said that her marriage was being offensive to God (Mark 6:18).  
 
Herod’s birthday party and his boastful promise gave her the opportunity she sought.  It was a macabre request to ask for John’s head on a plate.  Herod's pride gave him no way out but to have John killed during the party.  For Herodias, it was a success because the prophet was dead.  The girl was doubtless congratulated by her mother, and Herod felt he had at least kept his promise and proved to be a man of his word.  But he was not a man of God's Word.  All of them had failed to honour God and His prophet.
 
It is better to break a wrong promise to somebody, than to proudly keep it and break faith with God's Word.  Every rash promise is rooted in pride, fear or anger and opens a door to evil.  Only promise what you know is right, pleasing to God, achievable and sustainable with His help. Jesus said, "All you need to say is simply "Yes," or "No"; anything beyond this comes from the evil one.” (Matthew 5:37).  Promises stirred by a rebellious heart will end up hurting everybody.  It is good to keep your word if you have promised rightly and wisely.  However, if you have spoken wrongly, falsely or wickedly - you need to repent and seek mercy as you break an unwise pledge.  And do it soon, before the wickedness rebounds on you and hurts everybody else.