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.
The problem with saying she should have sought help is you're assuming she knew she was nuts. Part of the problem with insanity is from the inside it looks perfectly normal, the more seriously whacked people don't know they're not normal. You have to have some level of sanity to understand that you're in a version of reality that doesn't match the rest of the world, that's actually one of the most dangerous points in the life of the seriously disturbed, lots of people kill themselves when they start to realize just how nuts they have been, it's a very depressing realization.