A revenge tragedy, made popular in Elizabethan and Jacobean Britain, typically tells the story of a protagonist seeking revenge against the murderous actions of an antagonist. Shakespeare's Hamlet is probably seen as the most typical revenge tragedy of those times and the play that popularized many of the genre's conventions, including soliloquies, madness, action-packed scenes, bloody murders, important noble figures, suicide, and the use of disguise.
The Duchess of Malfi is also seen as a revenge tragedy, just not a typical one.
It is a revenge tragedy because, firstly, it features a character, Bosola, who seeks revenge for the murders of the play's most noble characters. Secondly, it features some of the genre's most typical characteristics: soliloquies, sensational murders, madness, and Machiavellian characters.
Bosola is the only character in the play who gives the audience insight into his true state of mind by speaking his thoughts aloud. For example, at the end of act 4, he shows guilt for arranging to kill the Duchess of Malfi by telling the audience,
All our good deeds and bad, a perspective
That shows us hell! That we cannot be suffer'd
To do good when we have a mind to it!
This is manly sorrow
Perhaps the most sensational murder of many is when Bosola orders the executioner to kill the Duchess and her children at the end of act 4. Just before the executioner strangles her, the Duchess says,
What would it pleasure me to have my throat cut
With diamonds? or to be smothered
With cassia or to be shot to death with pearls?
I know death hath ten thousand several doors
For men to take their exits. . . .
Dispose my breath how please you.
With the Duchess dead, Bosola tells his men,
some other strangle the children.
Like Hamlet loses his mind after the death of his father, the Duchess's brother Ferdinand loses his mind in act 5, scene 2 after the death of his sister. At one point he attempts to throttle his own shadow.
Eagles commonly fly alone: they are crows, daws
and starlings that flock together. Look, what's that follows me?
I will throttle it.
The Cardinal is probably the play's Machiavellian character and, though professing to be a man of God, kills his lover by tricking her into kissing the cover of a poisoned bible:
Now you shall never utter it; thy curiosity
Hath undone thee; thou 'rt poison'd with that book
Because I knew thou couldst not keep my counsel
I have bound thee to 't by death
Where The Duchess of Malfi is different from most revenge tragedies is that the protagonist revenging the deaths of the principle characters, Bosola, is the one that either killed them or played a hand in their deaths. Not only that, but he doesn't decide to take revenge until the end of act 5, scene 4, when he says,
I have this cardinal in the forge already;
Now I'll bring him to th' hammer O direful misprison.
Up to that point, the play could just as easily have been a tragedy in the vein of Romeo and Juliet.