...but that consistency is worth less than non-lawyers generally assume, for the reasons you bring up and more. And some states still follow other rules. On state levels, Supreme Courts base their judgments on the established law for that state, and while they may review precedents from other jurisdictions (including Federal) the established body of interpretation for each state differs. And "mental illness," "incapacity," "insanity" and other such terms are defined in civil as well as criminal codes, sometimes in very confusing terms.
Expecting these complicated issues to be resolved via public policy and judicial interpretation is still pretty futile, and centering the discussion in that arena will perpetuate damaging stigma, promote unrealistic expectations, and result in a high probability of poor public policy full of negative unintended consequences.
Of course, the law MUST have an evolving vocabulary, understanding, and practice in connection with mental illness-- I'm not saying it shouldn't. I'm saying that the rest of us need to lift the discussion OUT of that arena, acknowledge the complexity, and apply it across the many areas where it's needed. The goal should be to build communities that deal positively with mental illness, enable people who suffer from mental illnesses to access help, and promote their recovery over the long term.
Apart from questions of blame, evil, criminality, etc.
But it's a big windmill, and I have a very small toothpick.
regretfully,
Bright