Casey did not enter a plea of insanity or mental incompetence or even mental deficiency so it is not up to the jury to decide her mental health.
You totally misunderstand. I didn't say that she was insane or incompetant, I said that she had mental issues. They're not the same thing.
Also, you are totally misunderstanding what those sort of pleas are for. Those pleas are for defenses where the defendant admits to the charges, not for explaining why the defendant reacts in an odd way.
The fact of the matter is that using superstition to try to divine a person's guilt by their reactions to a crime is quackery.
What I didn’t misunderstand is that you were using the alleged mental issues you perceive Casey as having in determining her guilt or innocence. These issues were not addressed in court and such are not evidence to be used in deciding the verdict. I believe more than one juror made errors similar to yours and the result is a guilty woman went free.