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.