Josephus, who is the source for almost everything known about the reign, describes a king increasingly consumed by suspicion of his own household. Herod had his Hasmonean wife Mariamne executed in 29 BCE on a charge of adultery, along with her mother, her brother the young high priest Aristobulus, and her grandfather. Later he had two of his sons by her, Alexander and Aristobulus, strangled at Sebaste in 7 BCE, and his eldest son Antipater put to death five days before his own death in 4 BCE. The Roman emperor Augustus is reported to have remarked in Greek that it was safer to be Herod's pig than his son, a pun that works because a Jewish household would not slaughter a pig.
That documented record is the background against which the second chapter of the gospel of Matthew places its account of the killing of the male children of Bethlehem under two years old after magi from the east enquire about a newborn king. The episode appears in no other gospel and in no other source, and Josephus, who catalogues Herod's crimes at length and had no reason to protect his memory, does not mention it.
Most historians therefore treat the massacre as unhistorical, a narrative constructed to place Jesus in parallel with Moses, who also escapes a king's slaughter of infants. A minority argue that Bethlehem was a small village where such a killing might have involved perhaps a dozen children and would not have registered in a chronicle of a reign full of larger atrocities. What both sides agree on is that the story was told about Herod because it fitted him, and that the fit was earned.