According to this article http://www.rawstory.com/rs/2010/09/virginia-execute-woman-100-years/ the woman had an IQ between 70 and 72, which clearly proves your point that she was mentally challenged.

I'll go on record as saying anyone who concludes that an I.Q. of 72 puts her in a diminished capacity (to the point where she didn't know what she was doing) is probably operating on a pretty low I.Q. level as well.