The Cardinal is making Malateste and Pescara swear that they won't watch over Ferdinand tonight, or come out of their bedrooms even if they hear a ruckus.
He says he wants them to stay put because he doesn't want Ferdinand to be bothered at all, but really he just wants to be sure that he can dispose of Julia's body in peace.
The Cardinal has arranged for Bosola to come pick up the body, and plans to himself that he's going kill Bosola once he's completed the jobs he's given him.
Plans to murder your minions are best kept in your head: Bosola hears him, and knows what's up.
Ferdinand the Crazy wanders in and then out, talking to himself.
Antonio comes in, saying to himself that he hopes he catches the Cardinal while he's praying, so that maybe he'll be in the mood to forgive him.
Bosola, not being able to see that it's Antonio, and, thinking he's some random murder, interprets his words as "I hope I'll kill that Bosola while he's praying, maybe the Cardinal will pardon me then."
Quickly, Bosola comes forward and wounds Antonio, only to immediately thereafter discover that it's Antonio, the very guy he was hoping to save.
Bosola, horrified at what he's done, takes this time to tell Antonio that the Duchess and his two other children have been murdered.
Why on earth he should tell the dying Antonio that his family is dead when he'd previously lied to the Duchess about the same thing is kind of confusing.
Antonio, finally knowing the truth, tells Bosola he just wants to die.
He dies, after he voices his final wish that his remaining son be raised away from the courts, and that his body be given to Delio. Basically, nobody can catch a break in this play.
Bosola takes his body to where he was supposed to put Julia's body.
Quick thing to note: this is the third time the audience has watched Bosola drag a dead body offstage. Just think about that.