A child-killer; from Herod the Great, who ordered the massacre of the babes in Bethlehem. (Matt. ii. 16).
“Oh, it offends me to the soul to hear a robustious, periwig-pated
fellow tear a passion to latters, to very rags, to split the ears of
the groundings ... it out-herods Herod.” —
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894