It always seemed to me that Shigeru (as well as Seichi), didnt particularly like the idea of being forced to hang out together in a restrictive environment (Seichi's house instead of them being being able to go out and socialize like normal kids), which would lead him to tease Seichi a lot in an attempt to ease things out. However Seichi wasnt really fond of that, yet like his mom, didnt do shit about it.
Because nothing was done to address these tensions properly on both ends, things went to shit.
I think the mom has some serious issues, and because nothing was done to deal with those feelings she apparently always had, she ended up basically dumping her crap on her own kid. Even If Shigeru was a bit of a dipshit, It doesn't really justify pushing him off a fucking cliff ffs. There are better ways to handle those things, and above all else, SHE IS THE ADULT.
So far im glad Seichi is voicing all these things he thinks were going on. While is true his mom seems to have always been miserable with her life in general, he shouldnt feel responsable because he is a literal child. Hopefully the cops pick on the weird undertones and justice is served.