From following this case in depth since the deaths first happened, although I agree with your idea, at the same time, honestly I don't imagine she wanted SP back. That's the strange part.
She was clearly fed up in the relationship, long since had grievances, had left SP several times when they were together, they had completely split up and were living separately. Being a devoutly religious man, maybe he wasn't willing to grant a divorce (can happen in some religions 'we're married in the eyes of god for life')? Did she want a divorce and had been told 'no', 'not yet', or 'when the children are 18'?
Was she trying to kill him because she just wanted shot of him? Maybe she perceived him as a bothersome irritant in her life who increasingly didn't do what she wanted and she had begun to feel disrespected and mocked by (not invited to 70th birthday / putting 'separated' on the tax form etc).
JMO MOO