A child-killer; from Herod the Great, who ordered the massacre of the babes in Bethlehem. (Matt. ii. 16).
To out-herod Herod. To out-do in wickedness, violence, or rant, the worst of tyrants. Herod, who destroyed the babes of Bethlehem, was made (in the ancient mysteries) a ranting, roaring tyrant; the extravagance of his rant being the measure of his bloody-mindedness. (See Pilate.)
“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.” —Shakespeare: Hamlet, iii. 2.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894