This is a very complicated question, with many different answers. On one hand, Isabella is standing up for her own principles when she refuses to have sexual intercourse with Angelo. On the other hand, she is essentially condemning her own brother to death. While Isabella claims that she is acting purely out of religious and moral concerns, she must have some thought for herself as well. She does not want to have sexual relations with Angelo, and she knows that she should not have to do that. However, her brother's life is at stake. Perhaps she is right to protect herself and her principles, especially considering that she believes in an afterlife. But perhaps she is too cold and selfish.

Isabella believes that sacrificing her virginity would be a sin, but she claims that she would gladly give her life instead. Of course, this is not really an option, since Angelo has proposed only one solution. Remember that Isabella does not really approve of what Claudio did. She thinks the punishment is just, but hopes that Angelo might show mercy on Claudio. She sees herself as a virtuous follower of God; otherwise she would not seek to join a nunnery. It is possible that she would view her life as more valuable than her brother's in the eyes of God because she has not sinned as he has.

Angelo has certainly wronged Mariana, and yet she agrees to have sexual intercourse with him and marry him, not merely trick him. When the Duke condemns him to death, she asks him to reconsider, though the Duke tells her that she should be thankful her honor has been preserved and enjoy life as a widow instead. Mariana argues that men have faults and perhaps are even better for them. This issue is brought up a lot during the play, as the male characters sin and the female characters forgive them for it.

Isabella is understandably saddened by the false news of Claudio's death, and it seems unnecessary for the Duke to lie to her in this way, since he has hidden Claudio. This speaks to the play's general treatment of its female characters. They are not "in on" the Duke's plan, probably because he does not trust them to act properly without some manipulation. The Duke convinces Isabella that her brother is dead in order to make her angry at Angelo, probably thinking that otherwise she would simply let him be.

The characters in the play could, in most cases, be described according to single words or phrases. Isabella is virtuous and chaste; her brother is cowardly; the Duke is wise; Angelo is evil. Angelo is the only character that seems to have more complexity to his character, since he appears strict at first and then is shown to be hypocritical and somewhat emotional, allowing himself to be controlled by desire rather than rationality. The simplicity of the characters is a justifiable criticism of the play.

At the start of the play, Isabella is pictured as a completely non-sexual being, about to enter a nunnery and never speak to men again. This adds a twist to her decision, as she is clearly not desirous of sexual intercourse when Angelo propositions her. Angelo is so taken with her that he cannot resist asking her to have intercourse with him, despite the laws he is enforcing in Vienna. Later, the Duke falls in love with Isabella as well. Something about her is clearly very attractive to men. Lucio describes her as submissive and therefore very convincing. Perhaps it is her naive nature that attracts both men, who are obviously interested in power, given their government posts.

The play ends in marriage for Angelo and Mariana, Claudio and Juliet, and the Duke and Isabella. Even Lucio will probably be forced to marry a prostitute whom he has impregnated. This is a traditional ending to comedies, and it provides somewhat of a conclusion, at least suggesting that all the characters are about to embark on another phase in their lives. However, it is not really a happy solution for Angelo or Lucio, who would rather remain bachelors. Isabella's willingness to marry is unlikely, since she wanted to be a nun. Perhaps the implication is that Isabella joined the nunnery only because she could not find a worthy husband, just as Mariana did not marry because her only candidate left her. This says something about the treatment of women in the play, and their independence or lack of independence.

Marriage would seem to be a reward rather than a punishment, but in Measure for Measure it functions as a dreaded state for most of the male characters. In some way, the Duke's proposal to Isabella is a statement of his approval of the institution of marriage, his desire to give up his own possibly freewheeling life to settle down. Lucio certainly does not wish to marry. Claudio was engaged but could not resist engaging in sexual intercourse before the wedding; it is unclear how certain he was about his decision to take a wife. Angelo certainly does not wish to marry Mariana, since she has no dowry, but Mariana does want to marry him. Shakespeare was perhaps satirizing the institution of marriage and the bachelor's desire to avoid it if possible.

Claudio is told to resign himself to death, and both the Duke and Isabella offer him words of solace and encouragement. He is still frightened by the prospect and asks his sister to save him at the cost of her perceived virtue. Death is viewed as both a religious passage and the end to the joys of life, and Claudio is most concerned about his ignorance of what it holds. He is not ready to die, though he says he is. Even Barnadine, who the provost describes as being unaware of life or death, refuses to be executed. For Isabella, death is a better alternative than sinful sexual intercourse. The implication is possibly that no one is really prepared to die, however they try to convince themselves otherwise.

The Duke offers wisdom and assistance to Isabella, but much of it centers around demonstrating his own power and intellect. He could easily have disclosed his identity early on, freeing Claudio and punishing Angelo, yet he chooses to manipulate the situation through a different persona. He certainly enjoys carrying on his charade, needlessly complicating the decisions to be made and the solutions to the other characters' problems. Isabella is certainly devoted to the friar, following his every command. It is only natural that she should transfer her affection to the Duke. The occurrences of the play could be described as one long courtship ritual, in which the Duke emerges as a heroic figure by preserving a woman's chastity, tricking a villain, and saving a man from death.