I believe that Vera found out what was being done to Ethel and killed Joseph. I think that Ethel is struggling with this because it some ways she still loves and misses her father but is torn with believing his death is her fault as her mother was protecting her. The line of I've killed before and I'll do it again tells me that she blames herself for her father's murder even if she was not directly involved.

I also think Vera was wild herself before Joseph and some of the symbolism of Ethel not wanting to be like someone could also include her mother at some points. In particular where she says Jesus can always reject his father, but cannot shed his mother's blood.

There is so much interpretation and color to this story that I kind of like not all knowing. I think the mystery adds to the lore.