@flufflover36
It just doesn't make sense if Seiichi is the one who did all those violent things because all of the mother's actions afterwards were still abusive.
You've responded to your own point :
if Seiichi really is the crazy one then [...]why should we believe anything that occurred?
We've seen everything from Sei's point of view, to the point that we actually don't know what the other characters look like.
Strangely enough, it makes sense.
Seiichi is the one who killed the cat, but he told himself that she's the one who did it.
The creepy smile she always does is actually an horrified one.
When she look pleasantly surprised when he remember the dead cat, she was actually frightened because she thought he got over that.
His mother accompanied him to the kindergarten everyday because she was afraid of what he might do to other kid, he told himself that she's overprotective.
She wanted him to end his relationship with his girlfriend out of fear of what he might do to her, but he convinced himself that she's jealous.
Most of the stuff that she has done can be justified by her trying to genuinely be nice to him.
Then again, we don't know where this is leading up to.
For all we know, they both might be batshit insane, but Seiichi was pretending to be innocent.
Or we might all be wrong and the author will give us a truly unexpected twist next chapter.