The Duchess and Cariola hear some noises, only to discover that Ferdinand has transported all of the local lunatics to her living space.
The Duchess barely bats an eye: she's already endured the worst possible thing that can happen to her (believing that Antonio and her kids have been murdered), and explains to the Cariola that she's not actually crazy, she's just resigned to the fact that Fate is really, really stacked against her.
Her response to the servant who comes in and tells her she's going to have to watch a bunch of insane people cavort and scream around her? "Bring it on."
Then follows a scene that you basically can't summarize. Go read it, for full affect.
Basically, you get to watch the Crazy People Parade for a while, which consists of a bunch of lunatics saying dark, socially cynical (hey, people, this is Webster. We're all about the social cynicism) and scary things. While dancing and singing. Creepily.
They eventually leave, and Bosola comes in, dressed as an old man.
He tells the Duchess that he's here to make her tomb, and when she tries to explain, "hey, I'm the Duchess of Malfi, who the hell do you think you are?" He tells her, "yeah, I'm sure you're a noble lady, but moreover you're a soul trapped in a body, waiting for death."
The executioners come in with a rope, and the Duchess, seeing where things are going, assures Cariola that she isn't frightened.
Cariola, however, freaks out, and has to be taken away by the guards. Now it's down to Bosola, the Duchess and her executioners.
Bosola tells her that the executioners are here to strangle her, and the Duchess, without missing a beat, forgives them: how is being strangled by an executioner any different than dying by a stroke, or being shot to death with pearls? All death is the same to her, and at this point she just wants it over with, so that she can join her murdered family in heaven.
She kneels down, and the executioners strangle her.
Bosola orders the men to fetch Cariola, and to kill the children, who are apparently being kept somewhere else in the palace.
She tries every trick in the book to convince Bosola not to kill her: "I'm engaged! Wait, no, I'm pregnant! I have dirt on one of the Dukes!"
No dice: she's strangled with little hesitation.
Ferdinand checks in, and Bosola shows him that the Duchess is, per his orders, dead. Bosola tries to guilt Ferdinand, asking him why the children deserved to die.
Yeah, guilt is clearly not Ferdinand's thing—it's enough that the children were the offspring of the Duchess to order their deaths.
Then, Ferdinand does the unexpected. He tries guilting Bosola: "Why did you kill the Duchess? If you were a good guy, you would have protected her from me. All she did was remarry, and, what, we kill her for that? You're a jerk, Bosola, and I hate you."
Bosola is, understandably, kind of gobsmacked.
"Are you kidding me, buddy? You commissioned me to spy on her and then kill her. You know what, I'm done, just give me my money and we'll call it even."
Ferdinand, instead of paying up, offers to forgive Bosola for the murder. You know, the murder that he himself ordered and arranged.
Bosola is not happy. He says, "look, I'm not a bad guy; I hated all that stuff I did for you, but I wanted to serve you well, so I did it."
Ferdinand could care less about Bosola's moral qualms, and leaves.
Bosola is in the middle of proclaiming how he's a changed man, when, out of the blue, the Duchess stirs.
For one beautiful moment, everyone (including Bosola) thinks that the executioners failed Strangling Class, and that the Duchess is going to make it.
Bosola's elated—maybe he has some recourse to forgiveness, after all.
The Duchess blearily asks after Antonio, and Bosola immediately tells her that Antonio's alive and that the Pope himself has reconciled him and her brothers.
And then she dies. For real this time. What can we say? This is a Webster play; life is harsh.
Bosola, knowing she's truly dead now, grieves for her, and promises to take care of her body and to hightail it over to Milan (which, in case you've lost track, is where Antonio, Delio and the Duchess's brothers are all hanging out at the moment).