He and Neville were next to the aga.
Neville's body was in the perfect position for Bamber to burn his back.
Neville put up most resistance and put up a 'tremendous' fight for life. So may have still had signs of life
The twins were shot 5 and 8 times all shots in the head, and were only 6. So unlikely to survive.
Bamber couldn't burn Sheila's back. She was supposed to have committed suicide.
The twins were lying on their backs. Bamber would have to move them.
Bamber couldn't burn everyone's backs. It would look too suspicious.
The burning instrument may not stay hot enough to be effective while being taken upstairs.
Bamber may not have had time to burn everyones back.
Bamber was satisfied June was dead. She had been shot several times in the head.
Bamber burnt Neville's back just before he left WHF as he had to exit the kitchen window and pass Neville. This is if he did not use the rifle head.
If Bamber used the rifle head on Neville before returning upstairs, he didn't want to burn it too much in case he damaged it.
Neville showed signs of life after each burn. So had to be burnt three times. No one upstairs showed signs of life after their 2, 3, 5 and 7 shots.
Bamber was shooting a dead or knocked out Neville next to the aga. Why not burn his back as well ?