Yotsuba initially partook in the bullying but regrets it Uichi killed himself. (This one is pretty obvious people cmon)
Fuu is very sus and likely the mastermind.
My 2 cents
This is so fucking stupid now
He was already a dipshit evil kid, there's no need to overdo it making it even more unbelievable
Dropping this harder than the author's parents dropped them as a kid