I'm not an expert by any means, but I'll try to explain. (I do want to point out that I'm not going to link any references like I usually do. This case was entirely too disturbing for me, and I'd rather not reread all the articles and such to properly site things.)
Yates was warned by her doctors not to leave her alone. He knew she was not capable of providing basic care to them, as well as the fact that she could be dangerous.
Andrea Yates had been hospitalized for mental illness on multiple occasions. She had attempted suicide at least twice. She had been diagnosed with psychotic hallucinations and major depression.
And he left her alone with the children.
There are news articles that indicate that he was controlling her, restricting her access to outside interaction. People have claimed he insisted on her homeschooling the children and encouraging her to become pregnant when doctors explicitly warned her not to have more kids.
If my doctor tells me that my wife would have psychotic episodes if she got pregnant again, I'd be making appointments for my vasectomy. Not fathering two more children.
Even if you disregard the stories about his control/religious issues, the fact remains that he left a mentally ill woman alone with her children against doctors orders and in all absence of common sense.
People have been sent to jail for much less.
Hope that helps explain my comments.
