I provided a bullet proof Shelia scenario.
According to you a murderous healthy fit 24 year old has problems performing a coup de grace upon his 61 year old, bad back suffering, sleeping, bewildered dad
With a loaded rifle!
Not according to me.
The evidence is Bamber shot Nevill twice in the face and twice in the torso upstairs.
Nevill then got downstairs where Bamber brutally attacked, burnt and shot him.
Not sure why you are asking me why that happened.