The Hebrews too, looked to Rome for perpetration of the worst offenses against them. But we know that most abuse happens in the home, done by family members, by those the child trusts. Allied to Rome but not a Rome, he ordered this heinous deed not under orders from Rome, but under his own evil will. Like all acts of abuse, the massacre was an act of force, arising from feared or anticipated helplessness seeking the power to hold the terror off. As in all abuse, the victims were innocent.
Many hypotheses are possible.
Here are some. You may have many more. The massacre wiped out an entire generation of boys in this one place. Just as the Baby Boom has created a lump throughout our society, this destruction of infants created a vacuum. For as long as the people survived, there would be no males living who might have been born in those years.