- Also, there's a sort of opposite thing for witches going on: In Madoka, a witch is created when a magical girl falls into despair. In The Witch's House, falling into despair is what causes a witch to lose her immortality.
- Alternatively: the presence of a demon may indicate the presence of a Devil and a God in this setting. Hopefully, Viola is now peacefully in Heaven, while Ellen will have her soul eaten by the "cat" eventually, you know how those things tend to go for people who make deals with demons.
- Sadly, the Demon could very well have eaten Viola's soul when she was killed...
- Unfortunately, in the pseudo-third ending, if you investigate the spellbooks in the witch's room, Ellen (in Viola's body) will state "it's not so bad to be powerless for a while". This indicates Ellen might get her powers back sooner or later. And considering Ellen also obtained Viola's memories during the body switch and was desperate to be loved, Viola's father might not catch on so quickly after all.
- She was actually saved by the cat, and given a second chance at getting her vengeance. Although magic is hinted to be a part of Ellen, there's no reason why Viola can't learn about magic and how to use it in a witch's house. The magic will empower her still-dismembered form and she'll put all her magical effort to ruin the life of Ellen.
- She lost her legs, her eyes, and her voice, and was in immense pain. Really, at this point, it was a blessing.
- Why else would it need to be saved? Why else would you want to save it?
- Lets see: black cat demon, makes pacts with people, greets people with yo, eats the souls of children... literally the same character, how did they get away with this?
When we get to Mad Father, we see Ogre as the one letting Monika have her vengeance on the household. Not only that, but there's quite a few things in game that seem to deal with him. For one, Ogre is the one responsible for giving Dr. Drevis the tools and knowledge he needed to make his dolls. The Doctor is a doctor for a reason, and got that medical knowledge from Ogre. Finally, the save points in game are Crows, which brings us forwards to Misao.
In Misao, Ogre reappears as Onigawara, and here he's responsible for letting Misao have her vengeance on the student body of the school. He also conveniently serves as the only save point in the school, which if he is the Crow Demon, also fits in with the Crows being the Save points in Mad Father. Even more evidence of his connections to medical knowledge is that game is known for having quite a few callbacks to Mad Father. The main ones being Mrs. Library and the Laboratory section in the game, with a mad doctor wielding a chainsaw looking quite similar to Dr. Drevis. In the 2017 rerelease, he even has his private quarters that house dolls of all kinds, and Mrs. Library even rejects being called Aya as "Only father can call [her] by that name. As can be seen in some endings to Mad Father, Onigawara is responsible for setting the doctor back up there and continuing his work.
In short, Ogre/Onigawara could be the Crow Demon as described in the Diary of Ellen, and this is all the evidence from the various games that point towards it.