There are lots of scenario permutations. All which match the crime scene.
Bamber would have had a fixed plan. Based on his knowledge of WHF, the family & the rifle.
His ultimate aim was achieved. But the kitchen fight, back burning and Sheila's 2 shots were not planned.
He may have even attempted an 11 shot massacre, which quickly went pear shaped.
Despite this, his behaviour afterwards show he was still very confident. Having hours/days to peddle his narrative to the police would have helped.