How would I know the answer? You're applying logic to the actions of a woman with a head like a busted radio. And logic simply doesn't apply. That's one reason why crazy people are terrifying.
Why are you refusing to look at the facts? Do you prefer to believe she is crazy because she committed a horrific act?
A head like a busted radio?
She is a valedictorian - i.e. highly intelligent.
She is a trained nurse.
She hid her intent from her husband, friends and family.
She siezed a brief opportunity when she was left alone with
the kids to carry out her plan.