[Sherringford]

I'd just like to note, I was rereading S.S Van Dine's The Greene Murder Case and I can't help but notice how the murderer's insanity can rival Yasu's.

Spoiler for For The Greene's Murder Case:

Thank God I found a pdf of this, I was afraid I'd have to transcribe it.

""To trace the origin of her diabolical scheme we must first consider the locked library. Alone in the house, bored, resentful, tied down--it was inevitable that this pervertedly romantic child should play Pandora. She had every opportunity of securing the key and having a duplicate made; and so the library became her retreat, her escape from the gruelling, monotonous routine of her existence. There she ran across those books on criminology. They appealed to her, not only as a vicious outlet for her smouldering, repressed hatred, but because they struck a responsive chord in her tainted nature. Eventually she came upon Gross's great manual, and thus found the entire technique of crime laid out before her, with diagrams and examples--not a handbook for examining magistrates, but a guide for a potential murderer! Slowly the idea of her gory orgy took shape. At first perhaps she only imagined, as a means of self- gratification, the application of this technique of murder to those she hated. But after a time, no doubt, the conception became real. She saw its practical possibilities; and the terrible plot was formulated. She created this horror, and then, with her diseased imagination, she came to believe in it. Her plausible stories to us, her superb acting, her clever deceptions--all were part of this horrible fantasy she had engendered.""

Seriously, this would work as an explanation for Yasu.