I would thou hadst done so by Claudio. Go fetch him hither; let me look upon him.
I wish you had done the same for Claudio. Go bring him here; let me see him.
The Provost · Act 5, Scene 1
The Duke, learning that the Provost spared Barnardine's life, wishes he had done the same for Claudio. The line is poignant because it reveals that mercy, not measure, is the Duke's true intention all along. It also reminds us that Claudio dies because the Provost obeyed orders, but that his death is undone by theatrical substitution—the Duke's power to resurrect.
This nor hurts him nor profits you a jot; Forbear it therefore; give your cause to heaven. Mark what I say, which you shall find By every syllable a faithful verity: The duke comes home to-morrow.
This doesn't hurt him, nor does it help you at all; So stop it; leave it to heaven. Listen to what I say, you'll find every word of it true: The duke will be back tomorrow.
The Provost · Act 4, Scene 3
The disguised Duke, revealing himself to Isabella after she has been told her brother is dead, stops her from pursuing revenge. The line is remembered because it pivots the entire play toward resolution—the Duke stops the cycle of anger and begins to reveal his purpose. It also shifts the moral center: justice will not come through individual action but through the returned authority of the state itself.