At the end of the day, the choice was Bamber's.
He may have chosen beforehand not to go for an 11 shot massacre.
He would have thought 11 bullets into 1 man, 1 woman & two 6 year olds, was enough to at least be able to re load downstairs.
In the spur of the moment, Nevill may have decided to follow Bamber downstairs. Once downstairs he didn't have the strength to lay a finger on him.