Family murders were a recurring feature of top-level politics for centuries both before and after Alexander the Great’s time:
Consider the Persian Empire. The founder, Cyrus (reigned 559-530 BC, two hundred years before Alexander) unified the tribal chiefs in the highlands of Iran; conquered the major nearby states (Media, Babylon), then expanded west (into the Greek kingdoms and city-states of Asia Minor) and east (into the tribal zone of Central Asia). His methods sowed the seeds for the troubles that would come later. Where he conquered by force, he always legitimated himself as successor to the local kings or restorer of the local gods. Many of his accessions were peaceful, since his reputation preceded him and smaller chiefs welcomed him as a “friend”– which is to say, a political friend, involving obligations of military support and material or monetary tribute. In cities which had strong internal politics– such as the Greek city-states– Cyrus operated more by subsidy/bribery, resulting in takeovers that the losing faction regarded as treachery. Superficial political friendships were the prevailing manner; underneath was an atmosphere of side-switching and distrust.
When Cyrus the Great King died, the regions of the empire took the opportunity to revolt. Violence broke out in the royal family; the first son inherited and killed his brother, who ruled the eastern provinces, to eliminate opposition. This pattern would repeat itself, with a succession crisis after each death of a King. Sometimes the new King that emerged was weak, sometimes the process filtered out the weak and created someone strong.
The third King, Darius, rose from being son of a provincial satrap. He got his chance when the previous King, Cambyses, was away putting down a revolt in Egypt, whereupon an impersonator of the King’s brother was placed on the Persian throne. Cambyses died of a wound during the struggle to regain the throne; Darius joined the conspiracy to assassinate the pretender and emerged on top. Darius reigned 522-486 BC, spending his first two years traveling around the Empire with a mobile army putting down revolts, and started the first Persian invasion of the Greek mainland, which was turned back at the battle of Marathon (490 BC). After Darius died came the usual round of revolts, temporarily derailing the Greek enterprise, while his son Xerxes had to deal with revolts in the major provinces of Egypt and Babylon. Xerxes put together a huge army for another invasion of Greece, which eventually petered out through problems of logistics and maintaining connection via a mercenary fleet. Xerxes was assassinated in a palace plot in 465. His son Artaxerxes I ascended the throne, but his brother revolted (with the support of Athens– both sides could play the game of interfering in internal affairs of the enemy); eventually all the royal brothers were assassinated. Next came Darius II (reigned 424-405), an illegitimate son, who killed his illegitimate brother, who had killed his legitimate brother, Xerxes II, a murderous game of tag. Darius II married his half-sister, a cruel woman who controlled him and ruthlessly suppressed all enemies.
And so on. Artaxerxes III (reigned 358-338 BC) came in with the usual round of violence, in which several dozen brothers and sisters, nephews and nieces, and cousins of both sexes were killed. Now we hear of killing royal women; apparently connections on the female side were becoming increasingly important. Artaxerxes III was poisoned by his favorite court eunuch in 338 BC, followed by several years of royal family plots and assassinations; so many were murdered that the succession had to turn to a distant cousin. In the same year Artaxerxes III was murdered, Philip of Macedon won a great battle over the Athenian alliance (with his 18-year-old son Alexander leading the cavalry), making Macedon hegemonic in Greece, and setting the stage for the invasion of Persia. It was precisely the bloody succession crises that made Persia look like an easy target.
The atmosphere of betrayal and revolt spread into the administrative structure, even beyond the royal family. In the struggles and revolts under Artaxerxes II around 390 BC, a rebel satrap was betrayed by his son, who switched sides to line up with the victorious ruler, and had his father crucified. This sort of thing goes beyond family rivalry; it was a cruel public display of family hatred. Artaxerxes II is remembered mainly because his brother, Cyrus, with the support of his mother the former Queen, recruited the famous Ten Thousand mercenaries from Greece for an expedition to capture the Persian throne. This Cyrus died trying to cut through the bodyguards to reach his hated brother.