Well, if - if! - Richard Allen is proven to be the Delphi killer, here is your example. As average as can be, nondescript, local, living nearby, still, six years after the murders. This is what I mean. Either this, or someone whose “bisiness” makes him invisible but allows to get next to many houses.
I don’t dispute that a serial killer, or even a murderer, can be an average person. However, the difference with "our" killer is that he killed four people, which qualifies him as a mass murderer. This places him in a different profile category. As you might have read in the US press, the generalised profile of a mass shooter or mass murderer is rarely that of an average person. They may appear average, but there is usually something more distinctive about them, like you mentioned, an introverted nature, which is too broad a term.
In the Setagaya case, though, I think the murderer had a purpose when he broke in, was interrupted by Mikio, and then, overkilled because he was inexperienced and Yasuko was protecting her daughter. A mixture of intense anger and fear.
I think so, too.
Probably the behavior will be similar to that of the perpetrator in Idaho murders. Could, however, the fear on seeing own blood make the Setagaya killer hyperventilate and that caused dissociation? Maybe?
I'm not sure about that. I picture him as rather less perceptive and more basic than borderline. That said, I don't have a firm opinion on this. My focus is on id-ing and locating him, and what could help profile and find him.
About the temples that you kindly linked. Would any of them be accessible if he took a shortcut through the Setagaya park? Would they have bird feeders? Do you suppose that either could be located close to that place where Zelkowa trees grow in Tokyo?
I believe so, yes. The territory should be accessible, as well as the monk residences. I am unsure about the actual temple, though; it might be locked overnight.
Re: birds, good question. Buddhists indeed have something to do with birds. In some traditions, birds are kept in cages temporarily for ceremonial purposes. Like, for example there is a practice known as "life release" (fangsheng in Mandarin, Sōkō in Japanese), birds and other animals are released as an act of compassion and to generate positive karma. These birds are typically purchased from markets where they are sold for food or other purposes and are then released into the wild. It is believed that performing life release generates good karma and contributes to spiritual merit. Temples may hold special ceremonies dedicated to life release.
That place (surely not the only one) was at Seikei Uni.
Seikei University is a private university located in Kichijoji, about 10-15 minutes walk from Kichijoji Station. Established in 1949, the university is famous for its beautiful zelkova trees, besides being known for its high standards of education.
There is also a small Buddhist temple on the campus called
Kichijōji. There is also a museum of animation nearby:
Ghibli Museum is located at the southwest end of Inokashira Park. It is an art and animation museum of Ghibli Studio, one of the most famous animation studios of Japan. Owned by Miyazaki Hayao, the famous animation film maker, the museum features a variety of anime work and child-oriented fine art.