(Canto 12, lines 106-112)
The Alexander here is by some commentators believed to refer to the Thessalian despot Alexander of Pherae, famous for his conflict-filled relationship with Thebes and murdered in 357/356 BC, mentioned in the works of many of Dante's predecessors, like Cicero, Valerius Maximus and Brunetto Latini. However, it is perhaps more intuitive to think of Alexander the Great, who neither escaped the criticism of ancient authors like Seneca. From his ascendence to the Macedonian throne at twenty, he would conquer many lands in Africa and Asia and to classify him as "violent" does not seem all that strange. However, he was a student of Aristotle, and note besides that Dante seems more sympathetic to Alexander in other works. See for example the Convivio and De Monarchia.
Dionysius (d. 367 BC) was a famous tyrant of Syracuse, where he ruled for forty years.
Azzolin is Ezzolino III da Romano (1194-1259), Ghibelline lord of the March of Treviso from 1223. He became famous for his cruel rule and was even known as the Son of Satan - during his lifetime he was the most powerful lord in northern Italy. He was defeated by an alliance of cities (Venice, Ferrara, Padua, Cremona, Milan) supported by Pope Alexander IV, was captured and died in prison.
Obizzo of Esti here refers to Obizzo II d'Este (d. 1293), lord of Ferrara, Modena and Reggio. It is assumed that he was actually strangled to death by his son and heir lord Azzo VIII - however, by some accounts Azzo was his "bastard", which is why the poem uses the word "stepson" here. Dante's perpetrating of this theory is by some taken as a sign of his hatred towards Azzo, whom he couldn't place in Inferno himself (Azzo died in 1308). Azzo is however mentioned again in Canto 5 of Purgatorio.