The killing of children, specifically sons or other possible usurpers to the throne (the case here) was relatively common, the biblical king David being one fine example. Herod was himself so well known for killing his children that emperor Augustus allegedly said “It is better to be Herod’s pig (οὑϛ hous) than his son (ὑιοϛ huios)” (Macrobius, Saturnalia, 2:4:11), because as a Judaean he avoided pork, but apparently not filicide.