Then Herod, when he saw that he had been tricked by the wise men, was in a furious rage, and he sent and killed all the male children in Bethlehem and in all that region who were two years old or under, according to the time which he had ascertained from the wise men. (Matthew 2:16)This was not out of line with Herod's historical management style. He had a history of cruelty and executing his own family members.
I learned from Wikipedia that the approximate number of children involved would have been twenty. The Jewish historian Josephus recorded many of Herod's atrocities, but did not include this one, perhaps due to the relatively small number of victims compared to more his more grand displays of violence.