Kyd uses many allusions to the classical world. The topography of the underworld he provides is directly taken from Virgil's Aeneid, with some minor modifications. And he borrows many plot conventions and some rhetorical devices—for example, stichomythia, or a dialogue consisting of line-by-line exchange—from the Roman playwright Seneca. He also seems to adopt a pagan idea of revenge and justice: that humans must attempt to find justice for themselves (if they can), because the world full of injustice. There are also indications, however, that Hieronimo considers himself acting on God's behalf in his revenge.
Madness becomes manifested in two distinct persons in the play: Hieronimo and Isabella. The first case of madness eventually leads to bloody revenge, while the second leads to suicide. One turns outward for destruction, and the other seeks it inward. They are, however, both manifestations of a desire to escape from a horrible reality. Interestingly, the cases of madness are paradoxical, because they are a kind of "sane" madness—madness in the face of a world that has itself gone insane and to which madness is the only possible response. This madness places the sane and happy, such as the King, in an ironic position, especially if we understand "madness" as a disconnected state from reality. In the world of the play, it is the sane and happy who are truly disconnected from reality, unable to even see the pervasive evil that surrounds them.
An Elizabethan audience would easily recognize in Lorenzo, the chief antagonist of the play, the influence of Machiavelli, sixteenth-century Italian political philosopher. In Elizabethan England, Machiavelli's name was synonymous with evil. Though undoubtedly its impression of his philosophy was simplistic, Elizabethan England associated Machiavelli with duplicity and use of violence and fear. Machiavelli's philosophy was actually intended for the rulers of cities; he maintained (reasonably) that such rulers could not be bound by conventional morality. The Machiavellian villain however, of which there are many other examples in Elizabethan literature, applied the philosopher's principles to private life. Ironically, Hieronimo, the play's protagonist, is forced to adopt Machiavellian tactics in order to avenge his son.
Both rhetorically and in terms of characterization, Kyd loves opposites: Lorenzo is unequivocally unjust, while Hieronimo is unequivocally just. Horatio is honourable, while Lorenzo is typically dishonourable. This love for opposition expresses itself in the frequent occurrence of the rhetorical device of antithesis, where the opposition of two ideas is expressed in one sentence or in a parallel structure of sentences. But these antithetical structures will often culminate in a final sentence that resolves the differences between the two into an underlying similarity, either through a direct statement of this similarity, such as Balthazar's "I yield myself to both" or through an oxymoron, such as Bel-Imperia's "warring peace."
Similarly, many of the initially antithetical characters at times seem very similar to each other. At the end of the play, Hieronimo adopts Lorenzo's Machiavellianism, and Lorenzo plays Hieronimo's part of the innocent dupe. Because of Lorenzo's plot, the just Hieronimo ends up committing an act of injustice in the hanging of Pedringano. These resolutions and exchanges are ironic, because they show how both meanings and intentions are ambiguous and easily reversed: Bel-Imperia's love is both war and peace; Hieronimo needs to be a villain in order to be a hero and avenge his son; Bel-Imperia's desire to revenge herself on Balthazar by causing him pain ends up causing her intense grief; and the commission of justice can often turn into a commission of injustice (for example, in the case of the hanging of Pedringano). Such ironies pervade the play and help create the double perspective in which we view the action. We are separated from the actions of the characters, especially Hieronimo, by the knowledge that they act in error, but we also empathize with them because of the uncertain situations in which they are forced to act, in which the meaning and intentions of their actions often slip away.
We have, in addition to the play, a character within the play who watches the play's main events and is as isolated from them as we are: Don Andrea. We also have another character, Revenge, who—while separated from the play—seems to be affecting it in spirit and to have a knowledge of what is to come. He uses this knowledge to continually tease Andrea. We see ourselves in a very similar position at times, to both Andrea and Revenge, knowing what is going to happen and then not knowing, isolated from the action and yet identifying with the characters to whom it happens. The existence of this meta-theater thus serves to make the relationship between the play-world and the real world ambiguous; on one hand, we are still separated from the characters by a radical divide (we exist, they do not), but on the other, we exist in a position almost exactly identical to Andrea and Revenge. This ambiguity is played upon and further heightened by Hieronimo's revenge playlet in Act IV.