There is no accounting for crazy.
I also see a difference btw TH and MD. Though the distinction for me is a slight one. However, in the case of MD, he was her biological child and her ultimate decision hinted at a tiny bit of remorse. She even purposefully remarked before she checked out that her son "was never going to be found." She knew she had disposed of him sufficiently that she most likely couldn't have ever been charged. So I don't think she went into that closet because she felt her back was against the wall wrt LE. With TH, if she's responsible (and I think she is), she'll never feel remorse for her actions regarding this particular child. I think she likely feels justified to this day.
The more I think about it, the less likely it seems she would hurt herself. She's probably plotting her moment in the sun.