She never knew her place

The nanny shouldn't have been hired in the first place. She has always been unfit of the position because instead of only thinking of serving the family, she in fact, had always wanted to "use" the family for her own revenge and benefit. Her mentality was wrong from the start.
And she is absolutely despicable, not for betraying Luna, which I can understand. But for doing to orphans exactly as had been done to her despite how traumatising it was. And for being ready to betray, even now while she's being lead to trial.