Spool or not?
I didn't think Spool killed anyone. Personally I think that Norman did it all.
From what we see of her I don't think there is much motive to actually go on a killing spree?! So I say Spool didn't do it. Meanwhile Norman was insane, had gone on a killing spree before (and didn't remember doing it).
I might be completely wrong. Spool might have been the one. I can't tell conclusively weather she killed anyone or not, because we never actually see the killers face. If she did do it, it raises several questions about weather or not she was from the planet Krypton, because she did a hell of lot of heavy body disposal.
You could say if she did X, then it raises plot hole Y. Similarly though, if it was Norman that also raises plot holes. There is the locked in the attic scenario. If he was in the attic and the chick was moving furniture around down the back stairs, how does Norman go down the back stairs to kill the kid in the cellar?
Perhaps he has a key to the attic, which sits above the door frame out of reach of the chick but within reach of Norman. Now how does the chick have the strength to move furniture down the back stairs without alerting Norman. So something is wrong with something in that area of the film. Like I said, if someone did X, it raises plot hole Y. So you just have to ignore that stuff rather than try to use it as a logical proof of who did what to whom.
The whole thing with this sequel is the question of is he or isn't he the killer, not weather he (or mother?) is insane like the first one. In this one he was clearly insane from about 5 minutes in.
He thought he heard his mothers voice from her bedroom. A sane person would know clearly that Mother is well and truly dead and that there can be no way that is her. If one is hearing her voice, or a voice like hers, it must be someone else. He didn't make that distinction right from the get go. Therefore we are being told he was bonkers right from the start of the movie.
So it becomes, we are told he is still insane and then kept guessing if he did the killings till the end. Then at the end he just goes ahead and kills Spool. That is saying ah ha!! it was Norman doing the killings.