The mark of Hell is that the sinners retain those earthly qualities that condemned them. Francesca loved Paolo at first sight, loves him now, and will never cease to love him. Likewise, Ugolino hated Ruggieri in life, viciously hates him now, and no amount of hate and suffering will ever satisfy his desire for more and more hate.
Dante's genius is further seen in the fact that while Ugolino is in Hell for being a traitor, he is, instead, presented not as a traitor but as one who has been betrayed. The horror of his action is mitigated by the sufferings of a father. This is the law of retaliation: Ruggieri becomes the savage feast for the man who died of starvation along with his four sons. The horrifying image of Ugolino's savage repast is always before us — from the moment that Ugolino lifts his head from the "skull and other parts of the brain" and cleans his mouth by wiping off the "brain" matter, using his neighbor's hair as a napkin.
He, then, recites his tender narration of the horror of watching his four sons die one by one of starvation. Thus Ugolino hates violently because he loved his sons so intensely. His hatred is so great because his love was infinite, and his grief is so desperate because nothing can assuage him. As he finishes his story, he returns immediately to the gnawing of the brains and the crackling of the bones beneath him.
Both Francesca and Ugolino recollect the past with the same words, they both express their grief, and they both answer Dante's inquiries about their fate, but one emphasizes the controlling beauty of love, while the other dwells on the savage emotions of rage and hatred.
