Commentary Purg VIII 97-102

We have seen this 'snake' before, in Inferno XXXIV, imprisoned for his arrogant assault on God.  Now we see him replaying his role in the Fall, licking himself in prideful self-absorption as he plans another assault, now that he has lost his first battle, upon humankind.  Both Sordello's urgency in interrupting Virgil's notice of the heavens' beauties and the fact that no opposition has as yet deployed its forces have the effect of creating uncertainty (and fear) in the naïve onlooker and in the reader.  This serpent seems dangerous indeed, as he was when he tempted Eve in Eden, but in fact is not, as we presently discover.