Quote 4
But the feelings that must accompany the death of someone as close to my heart and bound up with my own being as it was possible to be, I knew then, in the nursery of Eel Marsh House. (11.19)
It's not just betrayal that's made the woman in black the way she it. It's actual heartbreak. Sure, the accident was no one's fault—but she's desperate to blame anyone, and so she blames her entire community.
Quote 5
But it seemed most likely that only a blood relation would have given, or rather, been forced to give her illegitimate child for adoption to another woman… (11.64)
Arthur figures out that Jennet's family betrayed her: the people she loved wouldn't let her keep her child. That's got to sting. (Or worse.)
Quote 6
Her passionate love for her child and her isolation with it, her anger and the way she at first fought bitterly against and finally, gave despairingly in to the course proposed to her, filled me with sadness and sympathy. (11.65)
Jennet thought that giving up her kid was the right course of action, so of course she feels utterly betrayed when he dies through what she considers negligence. We're pretty sure the phrase "grizzly mom" was invented for just this reason.