Because June crawled and died on the carpet?
Nevill left the bedroom within seconds.
Simple.
Let's say that a resolution of this problem depends on Nevill bolting from the room.
Steve says that Jeremy shot June in the head while she was still in bed. That means Jeremy needed to be stood at the foot of the bed, facing Nevill and June, and he would have been blocking Nevill's exit from the bedroom.
This also means that Jeremy must have shot June first, allowing Nevill to wake and escape, even though he had ample time to simply kill Nevill while he slept. Jeremy was using a semi-automatic rifle and just needed to point the muzzle end at Nevill. Nothing could be simpler.
Nevill has managed to get out of bed and past Jeremy, leaving no blood, despite being shot several times.
If the transfer blood mark on the wall of the landing at the door of Sheila's bedroom is from Nevill, that means Nevill has run on the opposite direction of the stairs, then stopped and run to the stairs. He has then only left one further transfer blood mark anywhere: on the wall of the first stair landing. All this time, Jeremy has not fired on him or caught up with him.