'Why on earth would any woman put anyone else before her children if she is mentally sound? It just goes against everything. '
Please, again, having self-esteem and co-dependency issues is not the same as having a mental illness.
Sure, it's possible for someone with a mental illness to have low self-esteem and co-dependency issues, of course, but it's not automatic that someone with those issues is mentally unsound/ill.
Assuming that someone, regardless of gender, who murders his/her own children is mentally unsound or ill is again, a rather insulting presumption to people who are, the majority of whom would never contemplate doing what Mrs Smith has confessed to doing.
People do put their needs above their children's all the time. For many other reasons than mental illness.
For example, not all those who are addicted to drugs and put their children at risk to fuel their habit are mentally ill or unsound.