That's the same rule that applies for liquor or other drugs that diminish the capacity and judgment of otherwise sane or capable persons. Given the "baseline" condition of this woman's sanity, I'm not sure how the law applies in reverse. I agree with you, but I don't know how the law stands.
In my experience, schizophrenics often know full well what they are doing and, at least when minor aberrant behaviors are involved, may use their conditions as an excuse. On the other hand, I'll never forget that one patient I was privileged to observe, in the company of his psychiatrist, who truly suffered from comparable (not similar but comparable) delusions to this woman's. Believe me, in his case, "suffer" was the operable term. I've thought of him often in the years since, and hoped that his change in meds helped him.
I still feel, based on the information given so far, that this woman was cognizant of the wrongness of her actions and should be punished accordingly.