King Herod was not so Great
The "King Herod" mentioned in today's gospel (Mark 6:14-29) is not King Herod the Great. Herod the great actually died several years before today's scene. The King Herod that had John the Baptist beheaded and had a hand in the crucifixion of Jesus is actually Herod Antipas, the son of Herod the Great.
What the two Herods have in common (besides being father and son) is that they were both narcissistic sociopaths. Power and influence corrupted them. They both felt threatened by Jesus and anyone who was associated with his movement in any way, including John the Baptist.
Today's passage is an example of how power can corrupt people. Mental illness and passions can lead us to do crazy things. Herod Antipas' ego was so sick that it craved the attention of his wife's, Herodias, daughter. If that doesn't sound weird remember that Herodias was married to Antipas' brother before she married Antipas! That right there sounds really sick too!
All of us whether we are in positions or not must keep our passions and desires in check. If we don't then we can do crazy things. We may not have an innocent person murdered but we can seriously hurt other people or their reputations.
Regularly examining our consciences and regular taking advantage of the sacrament of reconciliation can help us keep our egos and our passions and desires in check. That can lead to truly great things.