Commentary Inf XII 33

Wrath, punished in the Styx (the fifth Circle) is here distinguished from 'bestial' wrath that is the sign of a hardened will to do violence. As was the case in [Inf XI 83] (see C.Inf.XI.76-90) Dante uses the word 'bestial' to increase the heinousness of a kind of sin. Wrath is a sin of Incontinence; 'bestial wrath,' a sin of Violence; fraud is a sin of malice, 'mad brutishness' (treachery) a sin of greater malice. The reader should be warned that this view is not shared by many commentators. See C.Inf.XI.22-27, C.Inf.XI.76-90.