If you were to ask readers of Measure for Measure who they believed to be the most innocent character in the play , certainly Isabella would come up.  I would argue that Isabella is not as holier-than-thou as she appears to be. If we were to examine Isabel’s reaction to Claudio’s plea to save him, and compare that with Isabella’s plea to save Angelo, I see a bit of hypocrisy.

Claudio:

Sweet sister, let me live:
What sin you do to save a brother’s life,
Nature dispenses with the deed so far
That it becomes a virtue.

(3.1.133-136)

Isabella:

O you beast!
O faithless coward! O dishonest wretch!
Wilt thou be made a man out of my vice?
Is’t not a kind of incest, to take life
From thine own sister’s shame? What should I think?
Heaven shield my mother play’d my father fair!
For such a warped slip of wilderness
Ne’er issued from his blood. Take my defiance!
Die, perish! Might but my bending down
Reprieve thee from thy fate, it should proceed:
I’ll pray a thousand prayers for thy death,
No word to save thee.

It is understandable to say Claudio is asking for much. Specifically, for Isabel to go against her beliefs. Not only is she repulsed by Claudio’s plea, but she scolds him, by  saying what he did was a sin and that, “Tis best thou diest quickly.” Before we revisit her plea to save Angelo, we can recall her plea to Angelo to pardon Claudio:

I have a brother is condemn’d to die:
I do beseech you, let it be his fault,
And not my brother.

Here Isabel is asking Angelo for mercy, to “Condemn the fault and not the actor of it?” as Angelo put it. But she cannot understand that Claudio is asking for the same kind of act on her part, “What sin you do to save a brother’s life,”. As egregious as the sin may be, to save Claudio is it so inconsiderable, to bring Isabel to say to a brother waiting for death,  “Tis best thou diest quickly.” ? I cannot agree.

Finally, Isabella’s plea for Angelo’s pardon is a bit odd considering her rebuke of Claudio’s plea. After she kneels for Angelo she states, “thoughts are no subjects;
Intents but merely thoughts.” Isabel considers that Angelo’s poor intentions ought to be forgiven, but cannot consider acting against her beliefs for the sake of her brother. Her reaction to Claudio is what Harold Bloom called a “sadistic display of enraged virtue.” And I agree. I think Isabel is a very complex character. I’m not saying she should or should not have taken Angelo’s offer, or that Angelo’s proposition was right, but I found it interesting how she thought of mercy for others intentions, while mercy for her, if she done wrong, for the sake of her brother’s life, is unfathomable to her.