I was thinking about it. The Wikipedia says that the Japanese police thinks he is fast and impulsive, a risk-taker, but at the same time, sort of a planner given how he folded his clothes.
MOO. I wonder if his parents are very different people and maybe, the setup is the same as in Miyazawa’s family. I have seen some different photos of the family, and Mikio looks like most of our IT guys. And it would seem to me that according to the principle, “opposites attract”, he married a very vivacious, active, social woman with a strong character.
Yasuko was working. From what I understand, Mr. Miyazawa made decent money, and they had two young kids, but she had the cramming school, and she was the teacher, and many of them have some leadership traits. Who do you think wears the pants in the family? (And boy, was she fighting for her daughter’s life and her own, too. Amazing woman!)
Now if I could only imagine the murderer’s family, I’d guess his parents are of the same type, only way more polarized. The murderer has two sides to him. “A creature of habit”, a planner, he probably got from his dad, who may like routine and predictability. As to mom, I think she is very emotional, and maybe even more than that. But, dad has the place to escape, his work, and the kid stays at home with a strong, demanding mother.
Pure fantasy - I don’t know how, but I suspect via some connection, he was indirectly observing the Miyazawa’s family for a while. Rei, who is young and has some speech impairment (it is a hard moment in the life of any family, and the Miyazawas are proactive in treatment, but the stress on both patents is huge, and it is a difficult time for the family, no doubt). A very active, beautiful, vibrant mother, a quieter dad.
It could have been pure projection and transference. The perpetrator hates himself, but projects it onto the outside world, mainly, his parents. However, one cannot do anything with the parents. Maybe there was some quarrel between one of the Miyazawas and the bikers, and the perpetrator somehow snapped. Maybe he, indeed, blames his own mother more, or is a mere grievance collector. I don’t know if he has siblings or not, but if he does, he might be the younger of the two and maybe the older sibling is closer to the parents.
If I would take a guess, I think he dislikes both Mikio and Yasuko, each in his own way, and maybe he relishes in exerting control over the Miyazawa family. Lots of things he does in the house could be exactly what he is not allowed to do at home. Not flushing after himself, making a mess, eating more than one ice cream. I think he views Niina as a distinctive entity, as to Rei, he almost doesn’t see him, but if anything, associates Rei with himself. (“What’s the use of him growing up? He’d be like me”).
Covering Yasuko’s head means that maybe, he directed it more against the woman, but the obvious analogy with the matricide is scary for him and also, I think there is some shame because he saw how fiercely the mother protected her child.
He might be the middle child, too, and then Rei is the epitome of the younger sibling, whom he dislikes but doesn’t hate, and Niina, the older sibling.
All of it applies if he is an asexual/aromantic teenager. If he is 20 or over, lots of things may be changed depending on his sexual orientation, etc.