Randall Collins discusses the breakdown of democracy and the return of family murders in Rome:
The Roman Republic had a strong antipathy to kingship; when it came back, in the hereditary succession of the Caesars, its was accompanied by a renewal of family murders. The great founder and administrator Augustus kept domestic peace. He was personally very upright, moralistic, even prudish; but the younger generations of his family turned out just the opposite. Succession crises grew steadily bloodier through the next four rulers. It got so complicated you can’t tell the murders without a scorecard.
Augustus lived to be 77, and the only one of his children or grandchildren who survived him was his daughter Julia, who was so profligate with her lovers that he banished her and adopted as his heir a favorite general, Tiberius. Tiberius grew paranoid of conspiracies as his reign went on, resulting in a veritable reign of terror encouraged by his chief ministers, in which over 100 suspects were killed, and Tiberius’ only son was poisoned. Tiberius’ brother Drusus was long since dead, but his son Germanicus was another popular general; Tiberius had this nephew killed out of jealousy, along with his wife and two sons. To keep the lineage alive, the youngest of Germanicus’ sons was spared (only 7 years old at the time of these murders), and when Tiberius died (37 AD, at the age of 79), he had grown up to become the Emperor Caligula. Caligula in turn banished or murdered almost of all of his relatives except his incestuous sister; reigning only 4 years, he was killed by the Praetorian guard. The soldiers then put on the throne the only remaining member of the imperial household, Caligula’s old uncle Claudius, a retiring scholar, son of Tiberius’ brother Drusus. Claudius was an ineffectual emperor, run by his third wife Messalina– she married her own uncle. Notorious for her sexual appetite, Messalina also killed the daughters of Germanicus and Drusus, expanding the victim list from males to females to clear away potential rivals. Claudius finally had her killed when she married her lover in public. His next wife was even worse: Agrippina, one of the daughters of Germanicus that Messalina missed; she got Claudius to put aside his own son and adopt her son by a previous husband, Nero, as heir. That accomplished, she apparently poisoned Claudius.
Nero, finally, murdered Claudius’s son– just to make sure; then killed his own mother– the evil interfering Agrippina; followed by killing his wife, another wife, another daughter of Claudius who he offered to marry but was turned down, and took yet another wife after killing her husband. Finally the legions rose against him and he committed suicide, replaced by one of the generals. From then on, things moved away from family murders, into a pattern where the army generally decided the succession; in some periods of orderly rule the emperor would adopt a competent follower as his heir.
It was almost an experiment in all the things that can go wrong with family succession: aging rulers who outlive their own children; spoiled brats with unlimited resources surrounded by plotters; paranoia about conspiracies leading to preemptive murdering of all possible candidates, in turn resulting in even more unsuitable candidates, either completely unworldly types like Claudius or irresponsible pleasure-seekers like Caligula and Nero, Messalina and Agrippina. The amount of husband-murdering, wife-murdering, child-murdering, and even mother-murdering that went on match pretty much anything that the Greek dramatists could have plotted from their nightmares, without the dignity of projection into the world of myth.
I am aware that the structural conditions listed for the family murders in the Persian/Greek period do not all hold for the Roman period. The early Roman Empire was not a time of complicated multi-sided geopolitics; nor of exiles and pretenders at foreign courts eager to interfere. What was similar was hereditary rule based on personal loyalty, without checks and balances or administrative bureaucracy; and legitimate succession through marriages calculated purely for political advantage. The Romans seemed more reflective about it, in the sense that they looked for plots everywhere and engaged in paranoid purges to forestall possible rivals. Arranged marriages did exist in Rome, becoming quite prominent during the last two generations of the Republic, when the country went through repeated civil wars.
Despite the strong patriarchal structure, maternal connections became important; for instance Julius Caesar got his start by being related through his mother to Marius, the famous general and leader of the liberal faction during the Social Wars. Julius made his daughter Julia marry his various political allies, and divorce them when alliances changed. Since Rome did not have polygamy, divorce became common. Elite women got used to being important political actors—like Anthony’s wife, who carried on politics in Italy during his absences. The situation became more gender-symmetrical as the Empire was established; strong rulers like Augustus insisted that his favorite generals divorce their wives to marry into his family. This helps explain why there were women like Messalina and Agrippina. Roman women of the elite, unlike in Greece, were much more active schemers in their own interest; the nearest Greek equivalents were Alexander’s mother, and the last Cleopatra.
The classical Greek/Persian situation did reappear in some later instances, as in England during the time of Henry VIII and his 6 wives, in the midst of volatile foreign alliances and religious side-switching. It culminated in the struggle between two of his daughters, Queen Mary (“bloody Mary”) and Queen Elizabeth, the latter winning out via a series of revolts, plots, stake-burnings, and executions. Once parliamentary rule was established, murders inside elite families became a thing of the past.