Isabella enters Hieronimo's garden with a dagger. She is railing against the injustice of her son's murder. She then cuts down the trellis or tree from which Horatio was hanged, hoping to destroy the memory of her son's death. Unable to be apart from her boy any longer, she pleads with Hieronimo to join her in death. She then stabs herself and dies.
We are now in the rising action before the final crisis, or climax, of the play (the first crisis came in II.v, with the murder of Horatio). In other words, the central problem of the play since Act II scene v, Hieronimo's need to revenge of his son, now comes close to a resolution. The situation is almost an exact parallel of the rising action that preceded the first climax, except now roles have been switched. Lorenzo and Balthazar now play the roles of "innocent dupes." Hieronimo and Bel-Imperia are, by contrast, Machiavellian villains, plotting their revenge. As Hieronimo says when Lorenzo and Balthazar approach, "the plot's already in mine head." He then convinces Balthazar and Lorenzo to play a tragedy against the will of Balthazar, keenly manipulating Lorenzo's natural enthusiasm for a good play to his own ends.
The reversal of roles poses some problems in terms of the audience's sympathy for the characters. Machiavellian behavior has never been endearing, especially not in Elizabethan England. We must ask what the difference is, then, between Lorenzo and Balthazar's revenge and Hieronimo's revenge. For the murder of Horatio was also driven by revenge. One might say that that Hieronimo and Bel- Imperia are driven by love: Bel-Imperia's first question to Hieronimo is "Is this the love thou bear'st Horatio?" and she insists that she loves Horatio enough that she will kill his murderers if he won't. But if love were an adequate excuse, then Balthazar could be excused as well, because he committed his crime out of love for Bel-Imperia.
Hieronimo does however use the language of justice to describe his and Bel- Imperia's cause. "I see that heaven applies our drift / And all the saints do sit soliciting / For vengeance on those cursed murderers." The murders of Lorenzo and Balthazar, in Hieronimo's eyes, have heaven's approval on their side; the murder of Horatio was, by contrast, "causeless," unjustifiable. That Hieronimo feels his revenge to be a form of divine justice is clear, and it is also clear that the injury done to his son was much worse than any injury Horatio could have done to his murderers. Horatio's wrongs were merely personal slights, exacerbated by the murderers' keen sense of pride. So we have reason to distinguish between the two cases for revenge, even if the Machiavellian behavior of Hieronimo and Bel-Imperia makes us uncomfortable.
Hieronimo's deception has even fooled his wife, and this results in one of the play's saddest moments. Isabella's suicide is full of apocalyptic imagery: "I will leave not an herb within this garden-plot / Fruitless for ever may this garden be!" According to S.F. Johnson, her toppling down of the arbour (the tree from which Horatio was hanged) is an allusion to Hieronimo's prediction in IV.i: "Now shall I see the fall of Babylon, / Wrought by the heavens in this confusion." Her apocalyptic imagery has a common theme: the complete and total rejection of life, whether it be the life of the orchard, her own life, or the lives of any children she may have. This rejection is in effect a rejection of the future for the past, as shown by the juxtaposition of her suicide with the memory of her dead son: upon stabbing her breast, she describes it as "The hapless breast that gave Horatio suck." Her breast, and by extension her life, is given its meaning only by the memories of Horatio attached to it, not by anything that could happen in the future.