The actions that Hercules takes to defeat Cacus arise out of fury, but this fury is a necessary evil. It has provided Hercules with the will and strength to win the struggle. However, he still remains the gracious warrior that he always was. His fury is a reaction to Cacus's dishonorable actions, and therefore is somewhat justified.
Virgil is also able to alter the way that the passage containing Hercules and Cacus is perceived due to perceptions based upon the reading of previous passages of The Aeneid. Early in the poem Aeneas is repeatedly called "pious Aeneas", and there are many occurrences of his reverence. But even Aeneas has tendencies to lose his composure and let the fury inside of him out. As the city of Troy is under siege by the Greeks, Aeneas says, "In my mind a fire is burning; anger spurns me to avenge my falling land" (II, 774-776). Aeneas has let his fury overtake his piety when he attempts to stay and fight for his city instead of following his fate. But through his mother's intervention he realizes he is wrong to go against fate. Through this passage Virgil is able to show that even the most pious of people, such as Aeneas, have moments in which they let their furor overtake them. This alters the way that the reader looks at the passage with Hercules and Cacus, and allows the reader to recognize that although Hercules is overcome with furor in his struggle for Cacus, he can still be a good and pious individual.
Virgil also uses the passage with Hercules and Cacus to illustrate one of the underlying themes of the poem. Just as Cacus, who is full of furor and completely lacking of piety, is consumed, so are others in the book that are furious or impious. Turnus exemplifies this through his actions and the consequences of these actions. Turnus does not show piety throughout his appearance in the poem. He himself even denounces the Gods and the Fates by saying, "And I have my own fates to set against their own," and then again later, "I have no need of Vulcan's arms" (IX, 178-196).