I think you are placing too much empathsis on Nevill being able to do anything except flee.
The guy was being fired at. Even though awake he was still shocked confused and bewildered. Someone was firing a gun at him
Agree if he 'lurched out of bed' while June was being shot as CAL suggests, likely Bamber would be able to get 2 close range shots.
Bamber then had the option to physically attack Nevill but didn't want to risk getting an injury.
Nevill made it to the door & Bamber shot him twice in the back.
