This is exactly where my thoughts have been going.
For whatever reason, they did not want to take Z to the hospital. Being the [unusual people] they are, they decided to take care of the infection themselves. She was already missing part of her leg. She was fine after the first bit was removed. Just take a little more off.
They knocked her out with EB's pain pills or whatever. Took her to the bathroom or kitchen (I'm thinking bathtub) to 'take care of it.' Being the [people of undisclosed intelligence] they are, they didn't know about the femoral artery, and, she bled out.
I picture an 'oh crap' moment after they finished burying her and realized they still had the leg and prosthetic, and, that's why they weren't buried with her.
I can see EB rationalizing a situation like this as 'we really didn't kill her', 'we were trying to help her but she died'.
We'll see...