What's the Connection Between Bobby's Obsession With the Knife and the Aunts?

There are a lot of disconcerting signals about the aunt's kind of reliability or motivations. And i think one of the scary things about this story is that it's hard to tell if it's bobby's obsession with the knife or the aunts. It's another moment where the story gestures towards the idea that this be really a series of her projections of desires and terrors.