There were an awful lot of different sources weighing in on this, many with first hand information. Anyone who carves 666 into their scalp is a stark raving lunatic, and the physical evidence of that is such that it either happened the way various family sources said it did (i.e. she did it to herself) or dear Russell did it to her and she still didn't get herself and her children the he!! out of there.
Nobody can ever really know what her state of mind was at the very time she killed her children. People who are truly psychotic and/or schizophrenic have constantly changing perceptions. My half sister is schizophrenic (and due to our stupid laws. can't be forced into treatment until she kills or maims somebody). She lives in a makeshift tent near McAllen, Texas, despite my father's perpetual offers to buy her a little house. Some days she's very focused on her task of "guarding the border"; other days she's very focused on how she can't go into the post office to pick up mail from dad anymore, because the metal detectors that were installed at the entrance a few years back set off the things that "they" (somebody other than border-crossing invaders) implanted in her head. If she finally gets seriously violent and kills someone, there will be no way of knowing whether at the moment she killed them, she thought they were dangerous invaders from Mexico and it was her duty to kill them, or she thought they were the people who had implanted things in her head and they were about to try to implant more things, or thought some other non-reality-based thing, but we'll know she was out of mind when she did it, because she's always out of her mind to one degree or another.
Given Andrea Yates' well-documented history of complete insanity, the only reasonable assumption is that she was insane when she did it (and will never be remotely sane unless she's kept under close supervision so she can't deliberately or inadvertently not take her very strong medication). Whether the specific line of thinking that she'd expressed to relatives before, and IIRC repeated afterwards during psych exams, that the children could go to heaven if she killed them, was what was in her head when she killed them, we'll never know. Maybe she'd forgotten all about that, and had a different insane idea in her head right then. But the chance that she was thinking even remotely rationally is not plausible IMO.
I don't recall hearing about her carving 666 into her head, in the past, but just learned about it on this thread. You're right, it's insane. That's one thing I don't think Rusty did himself.
She listened to "sermons" which stated that women are sinful, they are bad, if they are bad their children will be bad and go to hell. Andrea internalized this message, and her mental state grabbed the ball and ran with it. She believed at the time that she was sending her children to heaven, before she had the chance to turn them into bad people because of her "sins". I have some experience about how bad religious beliefs can become ingrained and harmful.
I am so sorry about your half sister. It brings up what I said yesterday about laws prohibiting involuntary commitment. Extremely dangerous for all concerned, and they need to be changed. Your father must be heartbroken over this, as I'm sure you are as well :(
Yup, I have a schizophrenic relative (well, she's no longer alive).
susie