The Duke is still coming off a little like Prospero, making himself seem a benevolent advocate for Isabella, and one who used his power rightly, even when he was in disguise.But, though the Duke says he did "not [change] heart with habit," he did abuse his disguise as a priest to get people to trust him and believe in his unproven honor.
She tells the story of the contract between herself and Angelo, and that she slept with him in place of Isabella.
Angelo admits that he broke off a marriage contract with her, but was justified in doing so; Mariana begs the Duke for mercy, but Angelo says that he thinks these women are being manipulated, and should be punished for their testimony.
One ironic note, though, is that Angelo senses that the two women are "but instruments of some more mightier member"; in fact, they are playing the parts the Duke has set out for them, and this will soon be revealed to all.
Angelo's repentance seems a bit abrupt, especially his statement that he deserves to die for what he has done.
The Duke speaks in a grand, declarative tone when he re-enters the city; he has put on his public language and persona here, speaking formally, and glossing over what he knows to be the truth about Angelo's behavior.
The Duke's entering speech is laden with dramatic irony, since the audience knows that the Duke never left the city, knows everything that went on, and knows about Angelo's transgression; yet, as far as the citizens of the city know, Angelo has done a good job, and no one other than the Duke and his compatriots know otherwise at this point.
Angelo warns the Duke that he fears her wits are not about her; but, Isabella accuses Angelo anyway, of being "an hypocrite, a virgin-violator" before the Duke and his company.
She says that he appears good, but is notand that her claims must not be dismissed, but heeded with all possible caution.
He does this for no reason other to have her be very happy, and perhaps look even more just and heroic, when it is revealed that he actually saved Claudio's life.
The Duke did his duty as a ruler, seeing to it that justice was kept and no undue sentence performed; however, through this public performance of accusation, repentance, and grief, he shows the extent of his power, and makes himself look even more just, important, and beneficent since he appears to have single-handedly acquired justice for everyone.