King Herod existed during the Jesus’ birth. When he heard that a King is born he passed the draconian decree to kill all the male children.
As an idiom it means a Cruel Person.
Example: Stalin was a Herod when he put a large number of people to death during the great purges.
Hitler was a Herod as he tormented and put to death a large number of the Jewish people.
I am not a Herod.