He is writing 300 years after Alexander’s death but is still closer that Plutarch or Arrian. We think he may have based his account on Cleitarchus, who was writing fifty years after the assassination.
For Diodorus, the assassin was a bodyguard, Pausanias. Justin discusses a number of theories, implicating Olympias and Alexander. Plutarch names Pausanias, but says it was at the instigation of Olympias.