I can understand how you feel Caroline,,having realised that a mild-mannered and non-violent person can,,and does carry out murders,,albeit on a whim ( if pushed over the edge ), as opposed to any pre-planning or foreward thinking.
I doubt very much that if Jeremy had actually spoken about murdering his family,,,chances are,that he wouldn't have,,,especially having allegedly told both RWB and Julie beforehand.
When asked by Susan,,JM's friend, if it was Jeremy,,JM had answered no,,he got Matthew to do it,,so in a way she was still protective of Jeremy by putting the blame onto someone else,,,knowing in her own mind that Jeremy didn't do it.