With regards to the slap:
I have read manga where say the parent slaps the child for being egregiously disobedient. (It was more to really get their attention rather then a punishment). It's momentarily shocking, but I don't hate the parent as a result of it. (Though I strongly abject the use of slapping for any kind of punishment in regards with children)
Here, the reason (or the reason I can think of) for people being hostile for THIS mother doing it, is because we have experienced (along with Koto), 60 chapters of emotional abuse, so the slap is just the cherry on the top of this dumpster fire of a sundae we call the mother.
From the readers standpoint, the mother has created an emotional debt, where any positive thing she does will never be enough for redemption and any added negative thing will only serve to cement the opinion and fact that she is a horrible and weak human being.