I think she was insane at the time. I also think she is guilty.
We accept responsibility for what we do when we drink too much, though we are not in control of our facilities.
Diabetics are also responsible for what they do when they have low sugar blood, though in that state they have impaired judgment as well. They should carry around some food with them at all times if they wish to prevent this, despite the inconvenience.
Andrea Yates did not get there in a day. From everything I've read, it was a gradual process; she got there over the period of at least 5 years (the age of her oldest child, I believe), and clearly exhibited symptoms in addition to enough sanity during those years to function. She should've sought help, as was her responsibility to herself and to society. I would've voted guilty.