Pausanias was one of Philip II’s somatophylakes (bodyguards). He originated in Orestis, in Upper Macedonia.
According to Diodorus (16.93.1-94.4), Pausanias was Philip’s lover, but he became jealous when Philip passed over him to favour a younger man. His taunting of the new lover caused the youth to throw away his life. Unfortunately, the dead man was a friend of Attalus. Attalus took his revenge by inviting Pausanias to dinner, getting him drunk, then subjecting him to sexual assault.
When Pausanias complained to Philip the king felt unable to punish Attalus, as he was about to send him to Asia to establish a bridgehead for his planned invasion. He was also preparing to marry Attalus’ niece, Cleopatra/Eurydice (Diodorus is mistaken when he says that she was Attalus’ aunt!). Instead Philip tried to silence Pausanias by elevating him within the bodyguard. Pausanias' desire for revenge then turned against the man who had failed to avenge his honour; so he planned to kill Philip. (The details are basically the same in Justin 9.4.)
Whether this was sufficient motive for murdering the king, it is the only motive that the sources give us for Pausanias’ dissatisfaction; although it is possible that his anger with Philip made him a pliable tool for others—and suspicion fell on Olympias and Alexander from an early date. (Val. Max. 8.14 does provide another explanation for the murder; but it is extremely suspect—the same story occurs in at least two other places in the history of Alexander, as well as in other histories and biographies—it is where Pausanias asks how he can become famous, and receives the reply “by killing the most famous of men.”)
Plutarch (Alex. 10.4) says that Pausanias complained to Olympias, and that she incited the murder. He also says that some suspicion fell upon Alexander because, when Pausanias complained to him, he quoted from Euripides: “The giver of the bride, the bridegroom, and the bride.” (After gaining the throne, Alexander had Attalus killed, while Olympias slew Cleopatra/Eurydice, with or without Alexander’s blessing.) Justin (9.7) adds that Alexander was concerned that he would be supplanted by any son born to Cleopatra/Eurydice, and mentions his drunken quarrel with Attalus, during which Attalus prayed for a legitimate heir to the throne (the detail of the quarrel in Plut. Alex. 9.4-5). Justin credits Olympias with much more involvement, saying that she provided getaway horses for Pausanias, crowned his body after his summary execution, and built him a lavish tomb.
It is impossible to say whether Alexander or Olympias were really part of a conspiracy. If Pausanias did complain to them about his treatment they might well have sympathised, but felt they could do nothing to help; it is also possible that Pausanias might have taken their sympathy for encouragement to murder Philip, which they by no means intended themselves. Some historians have suspected Alexander’s involvement because three of his friends caught and killed Pausanias before he could talk (Diod. 16.94.4); although it is just as easy to explain that they were armed and on duty, and had just watched Pausanias murder their king—slaying him was an understandable reaction. The speed with which Alexander took the throne has also been seen as a sign of guilt by some; however, from the number of supposed rivals he executed at the start of his reign, and from what we know of Argead history, any delay on Alexander’s part could just as well have resulted in his death. So the fact that he acted quickly does not make him guilty of complicity in the murder.