ending interpretation

[S]

Excellent ending! Spoiler alert...what is your interpretation of the ending?

[Helen]

Martha did it

[Mrs]

I also think that Martha did it, as Jennie had no motive, she had Martha where she wanted her. Also, it says that Graham took care of the children and Sam came to visit - I don't think that would be the case if Jennie had killed Martha.
I don't know why Gillian White made the ending ambiguous though? It's frustrating and not really necessary. I understand why she kept it ambiguous at the start, to keep us guessing throughout, but why not make it a proper satisfying ending?

[Jennifer]

i kept fixating on the detail about how the victim kept her special knives sharpened. that's such a jennie thing to do - martha wouldn't care about that. so i thought jennie had to be the victim and martha the killer.

same thoughts about how she talked about the kids. it sounded like the killer was talking about martha's kids.