"An Angelo for Claudio, death for death!" he cries; "Haste still pays haste, and leisure answers leisure; / Like doth quit like, and Measure still for Measure" (414-16). Some critics have found fault with the duke and Shakespeare for letting Angelo off with little more than a warning for his heinous crime. Critics who interpret Angelo as a thoroughly evil man (not a fallen man of virtue) find his marriage to Mariana repellent. But, in fact, there is a certain ironical justice in the conclusion of his case. His crime is, after all, one of intent only; his intention was the rape of Isabella, but instead he went to bed with a substitute. For punishment he receives the duke's intent of execution, and only marriage with the substitute, in fact. An intended crime meets with an intended punishment, or measure for measure.
The duke, once revealed, tells Isabella that he could not prevent her brother's death because of the short time involved, thus reiterating his claim that her brother is dead. While she might otherwise have assumed that the duke had spared him, she still believes, when Mariana asks her to plead for Angelo, that he has been the instrument of her brother's execution. She remains silent through two lengthy pleas from Mariana, apparently struggling with her conscience, but finally makes her decision and pleads eloquently for Angelo's life. She does the Christian thing that she earlier asked Angelo to do on behalf of her brother: Judge not, that ye be not judged. She has said that if their positions were exchanged--if he were the supplicant and she the judge--she would show him mercy, and here she proves true to her word. In another earlier scene (II. i. 29-31), Angelo stated that, guilty of Claudio's crime, he would ask for the just penalty of the law, and he too lives up to his claim. Here in the last act, "No longer session hold upon my shame, / But let my trial be mine own confession: / Immediate sentence then and sequent death / Is all the grace I beg" (376-79). And again, "I crave death more willingly than mercy; / 'Tis my deserving, and I do entreat it" (481-82).
