What makes Dante's work so remarkable is his creation of the contrapasso - punishments in Inferno that ironically reflect or contrast the sin itself. We learn about this term in Canto XXVIII, when Bertran de Born states, "Thus is observed in me the counterpoise." In this, he is physcally separated by decapitation, and carries his talking head. He explains that his sin is a political one, by turning the Young King against his father.
"Know that Bertram de Born am I, the same
Who gave to the Young King the evil comfort.
I made the father and the son rebellious;
Achitophel not more with Absalom
And David did with his accursed goadings.
Because I parted persons so united,
Parted do I now bear my brain, alas!"
(XXVIII, 142)
The purpose behind the contrapasso is to serve a divine karma to the souls of the Inferno. Instead of generalizing the punishment unfairly, each soul is tortured as a reflection of their chosen sin during their life. For instance, located in the First Circle are unbaptized souls or virtuous pagans who are neither in Hell or out of it. While they have not sinned, they were unable to chose a side or accept Christ, which leaves them in a neutral area.
In contrast to this, Dante enters circles 2 - 9 and discovers more severe sins and their matching punishments. The outer ring of the Seventh Circle shows distinct differences between sinners that have commited violence. The punishment for violence is submersion in a river of boiling blood - symbolic for spilled blood - but highly violent tyrants are completely submerged, whereas those that commited less violence only sit ot stand in the boiling river.