Ahh, okay. I reread that part and realized I totally misread the whole scene. Maybe I was reading with my eyes closed the first time That person was the one whose clothes was disposed in the garbage fire, right? And they’ve been looking for her because she’s responsible of mysterious salty pieces of wood (for writing) or something like that Can’t remember the details.
I can hear the voice of my anatomy professor in the back of my head: “never ever tell yourself you can’t do something (or remember something in this case), the RAS system in your brain will believe you and in turn inhibit your performance”.
Once she gets into something, she gets completely crazy and doesn’t stop. So, if she has to do it (using dead bodies), she should do it last (i.e. when there’s nothing else she is curious about in the world).
怒っているようでいて means to look like one is angry. ようでいて is equivalent to よう alone but allows a connection to something else.
the そう makes it that it seems to her that [previous part]
でないそう is just “or not” (again, from her perspective). So she can tell if he is looking angry or not.