Obviously, Adam but no one is suggesting that, we are discussing the possibility that Sheila may have been his accomplice.
The whole thing is horrific, anyway.
Sheila as his accomplice makes no sense.
He didn't need her help to commit the massacre and stage the scene.
If he had asked for her help, to kill her family and herself !, she would have declined. It would have also given her advance warning and she may have told Neville or June.