Honestly her explanation (excuse) to why she did what she did and her telling her backstory doesn't make any sense...like girly pop how does it relate to their relationship? Yes you had a sad backstory but how does it connect to their relationship??
I was expecting some tie to the kid having a hard life because of it but that has nothing to do with her parents not giving a crap if she fell off the earth. If anything, her receiving adoration from outside the family counters any point that outsiders can fill the void of a loving family.
Her reason seems to be: I'm sad and you should suffer too.
(ノಠ益ಠ)ノ