The names of Hell's circles are taken straight from Dante's Inferno. This means that in terms of relative morality they represent the way a 12th century Italian intellectual saw things.

To Dante, treachery was the worst imaginable sin. Adultery was considered far worse a sin than the actual carnal activities themselves, and spies were judged to be far more despicable than murderers. Dante's choice of the ultimate sinners (three of them, each one being gnawed on by one of Lucifer's three heads) is also a bit surprising. Apart from Judas Iscariot, Brutus, the instigator of Caesar's murder might not strike one as the foulest of persons. And Cassius (Gaius Cassius Longinus, d. 42 BC), who was involved in the same plot to kill Caesar, is practically unknown these days. But these two guys had whacked Caesar, a man who enjoyed superhero status in medieval Italy. Most of us could probably think of a whole bunch of people more worthy to take their places, but this is Dante's version of Hell, not ours.