Adam Clarke on Matthew 2:20

They are dead - Both Herod and Antipater his son; though some think the plural is here used for the singular, and that the death of Herod alone is here intended. But as Herod's son Antipater was at this time heir apparent to the throne, and he had cleared his way to it by procuring the death of both his elder brothers, he is probably alluded to here, as doubtless he entered into his father's designs. They are dead - Antipater was put to death by his father's command, five days before this execrable tyrant went to his own place. See Josephus, Antiq. xvi. 11; xvii. 9.