Kaya-chan wa Kowakunai - Vol. 5 Ch. 31 - Language is not scary

[Retro68]

So her hatred was so vivid it manifested into an evil spirit and haunted her childhood bully's child?

I agree with this. I sincerely doubt the actual bully (the "perfect" mom lady) would even feel guilt for her bullying in the first place. I suspect it was done unconsciously on Mayumi's part.