Herod strangled his own sons. Both of them. On the same day. At Sebaste, the city where he had married their mother Mariamne twenty years earlier.
According to Josephus in Antiquities XVI, the final collapse began when Alexander was arrested on charges of plotting to assassinate Herod during a hunt. Under torture, a servant named Diophantus produced a letter allegedly from Alexander laying out the conspiracy. Alexander denied writing it. The court at Sebaste heard the case, but by then the verdict was already decided.
Herod had written to Augustus Caesar asking permission to punish his sons. Augustus advised caution and suggested a trial at the Roman colony of Berytus, present-day Beirut, before a panel of Roman governors. The trial proceeded without the accused present. Saturninus, the governor of Syria, voted for mercy. His sons sided with him. But the majority voted guilty, and Herod had the authorization he needed.
An old soldier named Tero made one last attempt to intervene, arguing that the army sympathized with the young princes. Herod had Tero and three hundred suspected sympathizers executed. Then he gave the order. Alexander and Aristobulus were taken to Sebaste and strangled around 7 BCE.
Josephus delivers a devastating judgment. He says Herod had every opportunity to spare them. He could have imprisoned them. He could have exiled them. He was surrounded by Roman legions that made any threat from two young men negligible. But Herod chose death, and he chose it deliberately, after months of delays that proved the killing was not an act of sudden passion but of cold, sustained malice. The king who rebuilt the Temple in unprecedented glory destroyed his own house with his own hands.