Self preservation
He will have no choice but to negate his daughter.
To avoid injuries to himself. He needed to go back to bed before work the following day.
Being a father
He will have no choice to negate his daughter.
To protect his daughter. He will know how to negate her as he was Sheila's father.
As head of the family he will be prepared to get injured in the process.
Being a grandfather
He will have no choice but to negate Sheila.
To protect his two 6 year old grandsons.
He would want to negate Sheila straight away. To prevent them waking and seeing their mother in this state.
As head of the family he would be prepared to get injured in the process.
Being a husband
He will have no choice to negate Sheila.
To protect his wife.
He would want to negate Sheila straight away. He would not want his wife waking & being in a stressful situation.
As head of the family he would be prepared to get injured in the process.
The danger of the rifle
He would know it was a rifle for shooting rabbits. He would know one accurate torso shot would not stop him disarming Sheila.
As head of the family he would be prepared to take one torso shot.
The danger of Sheila
He would know Sheila was uncordinated and had no experience with the rifle.
He would know he was twice as fit, big and strong as Sheila.
He would know he could use objects such as chairs to corner her, or disarm her when close enough.
He would know there was virtually no possibility of Sheila inflicting any injuries on him.
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All these would have been in Nevill's sub concious mind. As it would be in any father/husband/grandfather in that situation.
It would be an automatic reaction to disarm Sheila when he saw her holding the rifle. He would spend a few seconds deciding when to do this & may try speaking to her briefly first.
Ringing Bamber was obviously an impossibility as it would never be in his sub concious mind. It is also the worst option to him.