Going by the evidence -
STAGE 1:
Nevill enters the kitchen. Bare footed, in pyjamas. He has been shot 4 times. Bamber is a few feet behind him.
Evidence:
Nevill was found bare footed, in pyjamas. The bullet casings show he was shot 4 times upstairs.
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STAGE 2:
Bamber users the rifle butt to attempt to hit Nevill's head.Nevill puts his forearms up to protect himself.
Evidence:
Bruising and grazing on Nevill's forearms. Nevill's smashed watch found on the floor. Upturned kitchen furniture.
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STAGE 3:
To attempt to stop the blows, Nevill grabs onto the rifle. The two of them wrestle over it.
Evidence:
Scratch marks on the aga. Paint on silencer. Smashed ceiling light. Upturned furniture.
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STAGE 4
Nevill loses strenght due to his injuries. Bamber gets the rifle back and is able to get more success with his blows to Nevill's head & face.
Evidence:
Broken rifle stock. Nevill's black eyes, broken nose and 40+ head, face and body lacerations/bruises. Upturned furniture, broken crockery, sugar on floor.
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STAGE 5:
With Nevill knocked out, Bamber has time to reload & the strength to lift Nevill onto the coal scuttle. Then fire 4 more shots into Nevill. After everyone else has been killed, he burns Nevill's back.
Evidence:
Nevill was found on a coal scuttle with burns on his back and having received 8 shots.
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Obviously there is a chance they wrestled for the rifle first. However suspect Nevill was in defence mode after being shot and chased, while Bamber was fully pumped to attack. Nevill grabbed onto the rifle to try to stop the blows.