Measure for Measure Justice and Judgment Quotes

How we cite our quotes: (Act.Scene.Line). Line numbers correspond to the Riverside edition.

Quote #10

ISABELLA
Let him not die. My brother had but justice,
In that he did the thing for which he died.
For Angelo,
His act did not o'ertake his bad intent,
And must be buried but as an intent
That perished by the way. Thoughts are no subjects,
Intents but merely thoughts. (5.1.513-519)

When Isabella kneels on the ground and begs for Angelo's life, her logic seems a bit off (she suggests that because Angelo never actually slept with her, he shouldn't be put to death). The problem with this is that Angelo has slept with Mariana, and he's not married to her, which is exactly what Claudio did with Juliet. In other words, Angelo and Claudio have committed the same crime. So why does Isabella think Angelo should be exempt from punishment and that Claudio's death sentence was "just"?

This passage also makes wonder about whether or not Angelo's pardon (by the Duke) was just. Is it fair that Angelo is made to marry his ex-fiancé but escapes with his life? Why is the Duke merciful toward Angelo?