The main themes of the play are: corruption, misuse of power, revenge, deception, the status of women and the consequences of their assertion of authority, the argument of blood v. merit, the upshot of unequal marriage, cruelty, incest, and class.
A vein of corruption runs throughout the play. Perhaps the most apt representative of corruption is the deadly Cardinal, a man ready to employ lesser beings (such as Bosola) to commit murders for him, then cast them aside as rotten fruit. He is no stranger to murder himself, however, as he slays his own mistress by making her kiss a poisoned book. The acute Antonio describes him thus:
The spring in his face is nothing but the engend'ring of toads; where he is jealous of any man, he lays worse plot for them than ever was impos'd on Hercules, for he strews in his way flatterers, panders, intelligencers, atheists, and a thousand such political monsters. He should have been Pope; but instead of coming to it by the primitive decency of the church, he did bestow bribes so largely and so impudently as if he would have carried it away without heaven's knowledge. Some good he hath done.
He gambles, keeps the wife of one of his courtiers as a mistress, fights duels. Conspiracy and intrigue are the air he breathes. Duke Ferdinand is his brother's willing conspirator in villainy. At times, though, his rages shock even the cardinal's sense of decorum. The duke has a corruption that in the end destroys his sanity: incestuous desire for his own sister. Realizing she has married and borne children by Antonio, his rage drives him to do all in his power to bring his sister to despair, madness, and death, but in the end is driven mad himself.
Between them, these two perverse villains destroy or poison all that is within their reach, all semblance of warmth or human affection.
Misuse of power
The misuse of power can be seen in the Cardinal's and Ferdinand's actions. They make use of their power for their own greed and interest. The Cardinal abuses his ecclesiastical powers by getting Antonio's property confiscated and by getting the Duchess and her family banished from the state of Ancona. Ferdinand misuses his political power by ordering the death of the Duchess without any proper judgement passed by the court of law. (Ferdinand later blames Bosola for the murder of the Duchess by saying that he had no authority to get her killed privately since it was unlawful and Bosola could – and should have – helped her to escape). These are examples of corruption.
Status of Women and Responsibility for the Tragedy
The ideal quality her brothers would foist on the Duchess is that of being submissive to (their) male control, though ironically widowhood was often the first time women might be independent of the control of husbands or male relatives. However, the Duchess went against her brothers' wishes and remarried. Her assertion of her freedom of choice is best illustrated in her soliloquy following her conversation with her brothers when they strictly advise her to not even think about remarrying. Immediately after telling her brothers that she will never remarry, she says to herself: "If all my royal kindred/ Lay in my way unto this marriage,/ I'd make them my low foot-steps." The central conflict of the play involves the Duchess' desire to marry for love and her brothers' desire to prevent her from remarrying (either to inherit her estate and control her choices, or perhaps out of Ferdinand's potentially incestuous love for his sister). Throughout, she refuses to submit to her brothers' attempts at control and even asserts her identity and self-control at the moment of her death, announcing "I am Duchess of Malfi still" (4.2). Both her choices and her brothers' choices lead to their deaths and destruction, and the character of Bosola, who would protect the man he accidentally kills (Antonio), demonstrates that even though men would like to consider themselves authors of their own fates, at times, forces beyond their control intervene. Ultimately, and especially through Bosola's speeches, the play offers the dark view on the impossibility of making moral choices in an unstable world governed by the corruption represented by the Aragonian brethren.
The Cardinal and Ferdinand's cruelty towards the Duchess are evident in their threats. The poniard is an important symbol to show the threats. Cruelty is also shown in Ferdinand's wish to make the Duchess mad. He makes use of wax figurines to trick the Duchess into thinking Antonio is dead. Following this, he sends various madmen to the Duchess's room. This is to devastate the Duchess, in the hope of making her mad. The cruelty of their actions includes hiring Bosola as a spy, which deprives the Duchess of her privacy.
The Cardinal and Ferdinand are against the marriage of the Duchess and Antonio not only because they will have to share their wealth with him, but also because he is of a lower social status. Bosola, also of a lower class, expresses support for their marriage, at first, and even admires the Duchess for her ability to see past class; "Do I not dream? Can this ambitious age/ Have so much goodness in't as to prefer/ A man merely for worth, without these shadows/ Of wealth and painted honours? Possible?" (3.2.279–282). By the end of the play, his true feelings are revealed and he agrees with The Cardinal and Ferdinand that a steward is not a good match for the Duchess. The Duchess argues that high class is not an indicator of a good man.