Because it's been said that Sheila was drugged up to the hilt. Incapable of putting up a fight,a feeble female,etc etc,then why oh why didn't she gather both her sons and go to her own bedroom where the three of them could die together ? Even a killer would have worked that out as being more of a suicide plan than what had actually happened.
If it had been Jeremy,he'd have guided or taken Sheila to die with her sons.
If it had been Jeremy,they'd have ALL been dead by midnight that night------in their beds. Doesn't a gunman have the upper hand ? Of course they do,against 2 children and 2 pensioners and a feeble female.