The Inferno: Novel Summary: Canto 12

A sheer mountainside leads down to the Seventh Circle; they can only get down it where a landslide has shattered the rocks, but just there stands the Minotaur, half man, half bull. Virgil enrages him by taunting him with his defeat at the hands of Theseus, and they slip by while he writhes in rage. At the foot of the cliff is a river of boiling blood, in which all those who shed the blood of others suffer. Centaurs, half man, half horse, guard the banks, and they courteously help Virgil and Dante. The centaur who goes with them carries Dante on his back, so that he can get across the river, and points out the famous sinners who boil there. Deepest in the river are the tyrants and conquerors, such as Alexander and Attila, and recent tyrants of cities in Italy who had been notorious for their violence.
Dante does not speak of Alexander as Alexander the Great; the “glory” Alexander and his fellow tyrants won is lost in a river of blood, and they are distinguished from murderers and burglars by being plunged deeper in the river. As for the courteous centaurs, it has been suggested that many who at that time made Italy miserable by their violence seemed courteous and noble in person, but in Dantes eyes their violence made them less than human.