Good!
The thing is, I cannot see that he HAD to go with what he had planned, because it cannot have felt right to stage that suicide. I think the only thing a person could do was to leave the place unlocked and not stage suicide.
It wouldn't have felt right, to ME, to shoot all those people, either

so I guess that"it cannot have felt right to stage that suicide" is relative. We can't make this about us and what WE would have done. THIS is about Jeremy -so there's going to be some guess work- and HIS mind-set. If we accept -although I know you don't- that he was capable of carrying out this deed, the one thing I doubt he'd have felt at that stage, was remorse, but I DO think he'd have felt anger and irritation at Sheila for causing him to get it wrong. If he'd been throwing around, for the best part of a year, the idea of staging her death as suicide, I think the idea would have been SO entrenched, that shifting his mind-set, with any clarity of thought, elsewhere, would have been difficult...................although he may have been able, later and away from the scene, to have seen a better course of action.