Jeremy's supporters often claim he couldn't have controlled the occupants at WHF. But wouldn't the same be true for Sheila ?
So who was in the best position to control the occupants and carry out a successful massacre ?
Jeremy
He would have been fully clothed. Probably in several layers, with gloves and a balaclava to protect himself.
He was competent with guns.
He also had the element of surprise as everyone would be asleep. The perfect time for an execution. June & the twins being shot in bed while Neville escaped seconds earlier. Sheila had to be out of bed, so was either carried, lead of retreated to the far corner of the main bedroom. So not really much to control if everything goes smoothly.
If things got violent, he was a fit young man who could control two 6 year olds and a two women. Even if they woke. Neville was more problematic and the judge said he 'put up a tremendous fight for life'. Luckily for a fit, fully clothed and hyped up Jeremy, Neville was badly injured and eventually brutally beaten.
Sheila
She did not have any protective clothing. So more likely to get head, body or hand injuries.
She did not have the element of surprise. Neville being up and awake, and having the option of getting June for assistance or restraining Sheila himself. He even had time to spend several minutes phoning Jeremy and the police.
She had limited/no experience with guns. Holding a not very powerful rifle. She would have to accurately hit a moving Neville & a moving June, Nicholas & Daniel if they woke from the noise.
She was very slight and would not stand a chance in a struggle with a fit Neville or Neville/June. She may also struggle with an injured Neville or Neville/June and end up with injuries or losing control of the rifle.
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It is common knowledge that the crime was committed by either Sheila or Jeremy. It seems much more believable that Jeremy would be able to control the five occupants at WHF.