Why is this letter a threat to Villefort?

Villefort is about to release Dantes, yet when Dantes reveals to him to whom the letter was addressed he immediately changes his mind. The letter was addressed to Noirtier, who happens to be Villefort's Bonapartist father. Thus, Villefort must now try to cover the conspiracy to save his own name. He thus burns the letter, and tells Dantes to deny its existence.