Katy,
You are missing my point. You can follow that person's actions from A --> B
--> C, and it does make sense. I'm not talking about someone's mental state, I'm talking about cause-and-effect.
In your scenario above, if I studied this woman's motivations, I would come to a conclusion as to why it was done. It's never just because. There's always a reason. And, those reasons are discernible.
In the Casey Anthony case, the primary reason she was aquitted was because her lawyers were able to derail the rationality of CAUSE (silly, selfish, young woman who wanted to be unincumbered by a child for hours @ a time without letting anyone know it) --> EFFECT (young child overmedicated to the point of death) in the juror's minds. But, that is because our system of justice is not based upon what actually happened, rather what can be PROVEN happened.
Anyway, sorry to blabber on. Again, my mind works in that the most reasonable conclusion is the correct one.