the duke's castration

Act 3.5 

"Nay, faith, we'll have you hush'd now with thy dagger.

Nail down his tongue, and mine shall keep possession

about his heart; if he but gasp he dies;

We dread not death to quittance injuries."

In this scene, Vindici and Hippolito kill the Duke by poisoning him and then forcing him to watch Spurio with the Duchess. When they nail down his tongue, Vindici and Hippolito are castrating the Duke before they finally kill him. The tongue was a cause of many of the Duke's sins. By nailing it down, they are taking away his power of his choices and taking away his voice. He has a forced object into his mouth (the nail representing the phallus) and does not have the power to take that object out. He is now helpless and has to follow the choices made for him by men. In this scene Vindici and Hippolito take a cruel pleasure out of their violence. As Vindici called themselves, they all seem to be villains.