The bottom line here is that nobody knows what made Joan tick.  We can deduce  from her actions and her trial that she was a very complicated young woman, but  beyond that it's open to interpretation.  Was she insane?  - What about her  predictions?  Or possessed? - I for one can't even  imagine what that really means.  Perhaps she was just very impressionable  and instead of monsters under her bed she  saw angels?  That she was well  schooled in current political affairs I think we can  rule out.