Justin, Epitome of the Philippic History of Pompeius Trogus
But Darius, after such an extraordinary proof of his father’s affection, conceived the design of killing him. He would have been bad enough, if he had meditated the murder alone, but he became so much the worse, by enticing fifty of his brothers to participate in his crime - making them parricides too. It was miraculous that, among so many involved, the assassination should not only have been plotted, but concealed, and that of fifty children there was not one who respected their father’s dignity, or had reverence for an old man, or gratitude for paternal kindness; nothing could deter them from so horrible a purpose. Was the name of ‘father’ so cheap among so many sons? With such a retinue Artaxerxes should have been protected from all his foes; instead, surrounded by his treacherous sons, he was in less danger from his enemies than from his sons.
The motive of the anticipated parricide was even more atrocious than the crime itself; for after Cyrus [the Younger] was killed in the war against his brother. . ., Artaxerxes [II] had married Aspasia, the concubine of Cyrus; and Darius had required that his father should hand her over to him just as he had handed him the crown. Artaxerxes, typically indulging his children, said at first that he would do so, but afterwards he changed his mind, and in order to plausibly refuse what he had thoughtlessly promised, he made her a priestess of the sun, an office which obliged her to life-long chastity. The young Darius, being incensed at this, broke into an outburst of quarrels with his father, and subsequently entered into this conspiracy with his brothers. But while he was plotting his father’s destruction, he was discovered and apprehended with his accomplices, and paid the penalty of his crime to the gods who avenge paternal authority. The wives and children of all the conspirators were also put to death, to eliminate every last trace of this wickedness. Soon after this Artaxerxes died of a malady brought on by grief, having been more successful as a king than as a father.
The kingdom now passed to Ochus [Artaxerxes III] who, fearing a similar conspiracy, filled the palace with his family’s blood and with the slaughter of his most prominent courtiers. Nothing moved him to compassion - not family-bonds, not sex, not age. He simply did not want to be thought weaker than his murderous brothers.