Who assassinated the Persian king Darius III?

Bessus assassinated the Persian king Darius III in 330 BCE. Bessus was the satrap of Bactria and a senior Persian commander who turned against Darius as the Achaemenid Empire collapsed under Alexander's advance.

After the Persian defeat at Gaugamela, Bessus joined Darius during the king's flight east. Near Hecatompylus, Bessus and other conspirators arrested Darius and carried him in a covered wagon. When they learned that Alexander was pursuing them, they fatally wounded or killed Darius and abandoned his body, hoping to escape and continue resistance in the eastern provinces.

Bessus then claimed the royal title Artaxerxes V and tried to rule the remaining Persian territories from Bactria. Alexander used Darius's murder as a justification for pursuing him as a regicide and usurper. Bessus was eventually betrayed by his own allies, handed over to Alexander, and executed. Although accomplices participated in the killing, Bessus is the standard single answer to this question.