Amy gives a hell of a speech about cool girls. Very passionate. She really does feel she is wronged by the people in her life. We feel, for a moment, like she may be worth sympathy. Then you snap back. She is trying to frame her husband for murder. And then kill herself. Her villainy removes the sympathy, but it’s still there, in the back of your mind, like a sore you can’t stop poking at.
We do the opposite with our heroes. Someone did something great once. But now, come to find out, they also did evil things. Or at least bad things. Philandering, racism, hate. Now we step back. Does their wickedness take away from their great deeds? A (relatively) recent example: Thomas Jefferson. Enlightenment thinker, writer, revolutionary. Or, slave owner and bigot. Can a man so entrenched in slavery and the idea that blacks were less able than whites be forgiven in light of his achievements? Do the scales balance? How much good outweighs how much evil? How much evil outweighs how much good?
If Amy was a battered wife, would we feel she was justified in her plot, even a little? Does her description of how Nick hates her qualify as abuse, and therefore make us hesitate when we see Nick struggle to accept his possible incarceration and execution? Does any of it matter, in light of her strange sociopathic tenancies?