leitus or Cleitus the Black, was the Commander of Alexander's royal squadron of companion cavalry, Cleitus is a friend of Alexander's father Philip and according to Plutarch "Cleitus has a naturally surly person with a bad temper" which could be partly responsible for Cleitus's demise.
Clietus had saved Alexander's life at the Battle of Granicus (334 BC) where Alexander was personally under attack by Rhoesaces and Spithridates, Cleitus severed Spithridates's arm before the Persian satrap could bring it down on Alexander
Cleitus was killed by Alexander in 328 BC as a result of a drunken argument that got out of hand, the incident reveals stresses about Alexander's Oriental policies due to the initial argument was about the mistreatment of Macedonians compared to the foreigners that Alexander was "Placating" to.
what caused this drunken argument was that Alexander gave orders to Cleitus to take 16,000 troops of the defeated Greek mercenaries who fought for the Persian King before being defeated by Alexander and turning coats, to be sent north to fight the steppe nomads in Central Asia.
Cleitus saw this as a slight because he would no longer be near the King and perhaps even be forgotten, Cleitus voiced his frustrations and Alexander instead of trying to adhere to Cleitus boasted about his accomplishments and that they were far greater than that of his father (king Philip II), Cleitus responded to Alexander's remarks with something equally unwise in its approach by claiming Alexander was in fact not the legitimate king of the Macedonians, and his achievements were all because of his father and what he had inherited at this point Alexander called for his guards, unfortunately the guards were reluctant to intervene in quarrel between friends.
Alexander threw an Apple at Cleitus' head and then called for dagger or spear, but the group around the two men promptly removed any weapons from their reach and restrained Alexander and removed Cleitus from the room. The Hypaspists (royal guards) had left the vicinity of Alexander which at this point Alexander called for his trumpeter to summon the arm, the alarm was not sounded, Cleitus managed to return to the room and further throw insults and slurs towards Alexander, some sources disagree that Cleitus ever left the room but regardless at this point of the quarrel Alexander got a hold of a javelin and threw in it Cleitus's heart.
Alexander spent the rest of that night and the rest of the next day sobbing in deep remorse until his friend forced their way into the room where he lay in silence and exhausted Alexander paid no attention to what they had said to him except for when Aristander the diviner reminded him of the dream he had concerning Cleitus and its significance, told him that these events had long been obtained by fate, this seemed to stick with Alexander and accepted this assurance.
Despite this incident Alexander was determined to pursue his foreign policy, if anything the tragedy of that night spurred Alexander on to uniting his new nation even more so.