Nine times out of ten, Bamber would have been able to kill, or at least immobilise Neville while he lay in bed. The twins would have still been killed while they slept, before Sheila was woken and killed, or killed and moved. The five massacres being done smoothly within ten minutes.
This is what Bamber correctly envisaged happening and the reason why he attempted the massacre and frame.
This (tenth) time however Neville woke seconds beforehand. But still received four very damaging upstairs shots, two to the head from close range. This was enough to help give Bamber a decisive advantage in the kitchen fight