The Cardinal is walking about, contemplating hell and guilt and other stuff you think about when you're super evil, when in comes Bosola with Antonio's body.
Bosola cuts to the chase: "I'm here to kill you."
The Cardinal, knowing that killing people is kind of Bosola's specialty, freaks out and starts screaming for help.
Finally, something goes Bosola's way: Pescara, Malateste, and the other men are upstairs and can plainly hear the Cardinal having a conniption fit, but remember his previous order not to come even if he's screaming bloody murder, so they stay put.
After a bit, though, Pescara figures that something bad is actually going down, and so he ventures out to help the Cardinal. The others follow to watch the Cardinal laugh at Pescara's inability to follow instructions.
After killing the Cardinal's servant, Bosola wounds the Cardinal, who doesn't seem to understand why Bosola would want to kill him.
Bosola continues to stab the Cardinal when Ferdinand comes in.
At this point, things just become kind of farcical—Ferdinand both a) has no idea what's going and b) is still totally cray, and so after babbling a bit he both wounds his own brother further and, totally by accident, deals a mortal wound to Bosola.
Bosola kills Ferdinand, as the latter cries out about ambition and his sister and death. Geez, finally.
The noblemen upstairs finally get into the room, and are pretty shocked at what they find.
Bosola claims that he's completing his revenge for the Duchess, Antonio, Julia, and himself.
The Cardinal dies.
After painting a bleak picture of the world as a "shadow, or deep pit of darkness" (5.5.100), Bosola dies.
Delio comes in with Antonio and the Duchess's son, only to find that he's way too late to the Blood and Guts Party to be of any use.
He closes the play by telling the assorted noblemen that they have to try to make the best of the tragedy, and that they've got to establish the eldest son—now the only surviving member of the entire family—as the heir to his mother's position and her integrity.