Fair point. If she had killed puppies, everyone would agree on condemning her.
If postpartum depression is an excuse for murdering your children, then postpartum depression is an excuse for ANYTHING and EVERYTHING. Similarly, ANY mental health disorder is now an excuse for anything and everything. Are you really prepared to live in a world where this is the case?
Remember that mental health professionals and psychiatrists often don’t agree on the diagnosis of depression or anything else. It’s not like there is a blood test, an x-ray or an MRI.
We are essentially legalizing any and all behaviors and placing the fate of civilization–including your own personal physical safety–in the hands of paid experts who cannot even provide physical proof for their diagnoses. You will not be safe from violent criminals if they know they can walk free with a doctor’s note.