Respecting everyone's knowledge on this thread.
Would like to add...
I've worked in this field for a long time now and have seen a few really bad cases like this, where serious and consistent fucked uppery is present in the life of a child causing severe disturbance in their understanding of what's normal, let alone tolerating and doing what's normal. I don't always believe they have MH conditions, but they have been fucked up.
Sometimes it does cause for example schizophrenia. Too many of the people in MH hospitals and care homes were seriously abused for it not to be relevant, imv. Sometimes there's only so much a person can take before they change.
But sometimes it is the behaviour of the individual, their way of relating to people, how manipulative they are, how the trauma affects them etc that is what you see. She may not have had a diagnosable condition and the fallout may not have been immediately obvious, but how could you go thro her childhood and then plan and commit an act like that without something being wrong?
In this case I dont feel able to disconnect the woman and her crimes from the crimes against her, and it is obvious to me that, even tho we may not observe muddled thinking or psychosis (especially not in a safe environment) there would have been fallout evident in her behaviour due to the abuse perpetrated against her. And there was. Was she mentally ill? Something was going to be wrong wasn't it, after all that! It's not rocket science.
As a survivor of abuse myself, thinking this thro when I first worked in criminal justice just gave me extra impetus to sort myself out so I dont do the types things that were done to me. It makes me sad to see so glaring an example of an abuse victim that passed it on down. We have to do more for people who have suffered, for ourselves as much as them.