Again, Hieronimo is alone in his house, and now he has realized that he will not find justice with the king. He briefly considers leaving the matter of Horatio's revenge to God, as the Bible suggests. But he then considers that Lorenzo will probably have him killed to eliminate the threat of revenge, no matter what Hieronimo decides to do.
These considerations seem to prompt the Knight-Marshal to consider his vengeance against Lorenzo as part of his destiny, something fated by Heaven to happen. Now viewing himself as an instrument of divine vengeance, Hieronimo hatches a plan to pursue his revenge through subterfuge. Since both Lorenzo and Balthazar are of much higher rank than he, and could crush him easily if they knew his intentions, he will pretend to be grieving. If he acts unaware of Lorenzo's crimes and is friendly towards both of his son's murderers, when an opportune time comes for revenge, the two will not suspect him of seizing it.
A servant informs him that several petitioners are at the door, asking that Hieronimo plead on their behalf to the king. Hieronimo lets them in; they number four in total, and one of them is an old man. As they enter, the first speaks of Hieronimo's reputation as the most educated, skilled, and fair legal official in all of Spain. Hieronimo asks the men to plead their cases. The first three citizens all do so: the first case related to a debt, the second concerning some undetermined financial dealing, and the third an appeal of an eviction notice. The men provide Hieronimo with written documents and evidence, after which Hieronimo asks the old man to speak. The old man proclaims himself unable to speak his case, because it is too terrible; he instead provides Hieronimo with a document entitled "The humble supplication of Don Bazulto for his murdered son." Hieronimo is immediately moved into a fit of grief and shame over his own dead son and his inability to avenge Horatio's death. He offers the old man, who has been crying, a handkerchief, then realizes that it is the same handkerchief he pulled from Horatio's dead body.
He gives the old man coins and then goes into a diatribe in which he accuses himself of not grieving enough for his murdered son, not being a "loving father" as the old man has been. He vows horrible revenge, invoking the name of Proserpine, then runs off and tears up the legal documents of the various petitioners. They protest that he has gone mad. He runs out, only to return to speak to the old man. He asks the old man whether he is Horatio, returned from the dead to spur his father on to vengeance; the old man says no. He then asks the old man whether he is a Fury, come from the underworld to torment Hieronimo for not avenging his son. The old man replies that he is not a Fury either, but simply a distraught old man seeking justice for his murdered boy. Hieronimo then says that he knows what the old man is; he is the embodiment of Hieronimo's grief. The Knight-Marshal then asks the old man to accompany him into his house to meet Isabella, where all three of them will "sing a song" of the grief they all share over their lost sons.
Scene xiii gives a detailed portrait of Hieronimo's interiority—his inner psyche—that is both disturbing and profoundly moving. On the one hand, Hieronimo seems to accept his role as a Machiavellian villain. On the other, we are presented with an image of Hieronimo as an essentially just and kind human being, almost destroyed by grief for his son. And so the scene presents us with several questions of interpretation: these questions ponder whether Hieronimo is a hero or a villain; whether he freely chooses to act the way he does, or is forced into it by uncontrollable grief; and whether his act repudiates Christianity, or acts within it.
These questions are interrelated, especially for an Elizabethan audience. When Hieronimo enters and suddenly declares "Vindicta mihi!", he is quoting the bible: "Vengeance is mine; I will repay, sayeth the lord" (from Romans). Elizabethan preachers used this phrase to demonstrate how vengeance was something that should be left for God to invoke and not humankind. Hieronimo elaborates upon this Christian sentiment but then appears to reject it. The quotations he uses to explain his decision all draw from Seneca, and the book he holds is presumably a copy of the ancient Roman writer's works. But his reasoning is somewhat unclear. He proposes that Lorenzo will kill him to eliminate the threat of an avenger, if he does not kill him first. Another premise he states is that the worst that can happen to him if he acts boldly is death, which (since "Heaven covereth him that have no burial") is nothing to fear. But it is a logical leap from those two premises to the conclusion that he should reject the Christian injunction against revenge. Francis Bowers has suggested that we read this passage not as providing justification for Hieronimo's decision, but instead as an illustration of how he has turned from hero to villain. In other words, it shows his rejection of the Christian ideal of letting God decide and his adoption of the pagan idea of personal vengeance, as symbolized by the pagan writer Seneca. For an Elizabethan audience, the choosing paganism over Christianity and choosing evil over good were the same thing.
But this interpretation is somewhat inconsistent with the events that follow. These events emphasize Hieronimo's goodness—for example, the three citizens who praise Hieronimo's justice and fairness—and his anguish not only over his son's death but also over his reluctance to seek revenge for his son's death (as revealed by his conversation with the old man). An Elizabethan audience would undoubtedly feel discomfort with Hieronimo's decision, most likely believing it to be wrong. But it would also empathize with Hieronimo's suffering and his need to seek justice for his son. It must be remembered that all legal means to this justice have, at least in the eyes of Hieronimo, been exhausted after his failed meeting with the king. To Hieronimo, Lorenzo and Balthazar now have the law on their side, whereas Hieronimo has justice on his. Furthermore, private revenge had a long history in England, which had only fallen out of favor in the previous 100 years; no doubt Hieronimo was expressing an ambivalence probably felt by many in the audience. That they might not agree with the way he resolved his ambivalence does not mean that they would not sympathize or agree with his decision.
Furthermore, his decision does not amount to a wholesale rejection of Christianity. His reference to "Heaven" is an indication that he does not consider his actions as contradictory to Christian salvation. The stealth and deceptiveness he says he will pursue of course have a Machiavellian tinge. But they can also be interpreted as the patience of the righteous Renaissance man, as noted by Ronald Broude, or as the slow, patient revenge of God, described by the Protestant Theologian John Calvin, whose theology deeply influenced the English church. Hieronimo thus has an ambiguous relation to Christianity, where he may be usurping God's role or acting as God's agent. Even if he acts as God's agent, this is no guarantee that he acts justly.
It may be tempting, in the face of all this ambiguity and contradiction, to declare Hieronimo insane, and view his actions as those of a madmen. As for his sanity, Hieronimo seems to be hanging onto it by a thread, if at all, as demonstrated by his running away from the citizens and his hasty reasoning in his soliloquy. When he demands of the old man whether or not he is Horatio, or a Fury from Hell, the audience may feel that Hieronimo has lost it, but this tension is relieved in his final speech to the old man, where he describes him as "the lively image of my grief" and asks him to come and grieve with him and his wife, to sing a song "[t]here parts in one, but all in discord framed." It is an act of profound understanding and compassion, which encourages us to see Hieronimo not as a bloodthirsty villain, but as a noble man under a great deal of strain, who we find it difficult to condemn, even though he makes what audiences both Elizabethan and modern would mainly regard as the wrong decision. Again, we are forced into a double perspective, unable to either condemn Hieronimo or support him, instead both sharing his pain and recoiling from the action he is about to undertake.