It's just like Fumi-sensei said in that interview. Sometimes you look back at the things you liked in the past and fall in love with them all over again, bringing some light back into your life.
I mean at her age, other than a knife or pushing them into a train or so i don't think a 13? year old would be able to kill her parents with a golf club versus poisoning anyways
Tho given that this is 'semi autobiographical' hopefully she got to talk to some therapists or so