It seems that Neville was expecting Bamber to attack him. Telling BW he was worried about dying in a shooting accident, and that he 'must never turn his back on that young man'. AE said Neville and Bamber had a blazing row just before the massacre.
Neville was not expecting Bamber to go after the entire family. Otherwise he would not have not have invited Sheila and the twins over. If Neville was expecting Bamber to do this, he would know his only opportunity would be a late night execution attempt, when everyone was asleep. Neville would have taken precautions and secured all doors and windows with bars and locks before Sheila's visit.
What were the advantages of Bamber just killing Neville ?
He didn't like Neville.
He resented the way Neville treated him in comparison to Sheila.
Just killing Neville would be easier than killing five people. He would have more chance of succeeding and avoiding prosecution.
Killing just Neville, he could say it was a shooting accident. Although it would have to be a shooting accident with a shot gun. Shooting someone 8 times is no accident.
He didn't like June. Killing Neville would upset June.
He would not inherit all of the farm. However Sheila and June had no farm skills or interest and the twins were too young. He would be running the farm.
The twins were 6. So would not take any interest in WHF for at least ten years.
Bamber may have some morals. Just killing Neville means he would not have to live with himself knowing he had killed women and children.
He would be in charge of the day to day running of the farm. No longer a farm labourer and told what to do by Neville.
He owed Neville a lot of money. The debt would be cleared upon Neville's death.
He may have more of a share of the caravan site.
Neville would be dead before he could amend the wills to include the twins more. Something June was thinking about doing.
Sheila was too ill and disinterested to have any inpact on WHF. She may inherit half of Neville's will, but Bamber felt he could control her sufficiently.
Financially he may inherit a decent amount. Maybe even enough to live the life he wants.
Neville's will said Bamber had to continue working on the farm in order to inherit. If he inherited a good amount after killing Neville, he may not have to do this.
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As it happened, Bamber went after everyone. And succeeded in killing them all. And nearly succeeded in getting away with it. Was Neville right to just expect Bamber to be interested in him ?