Hi Grahame
I can't imagine a hit man in what we term as a hit man...one who goes in goes a professional job then out.
Those murders are not done by anyone professional are they?
Not one bullet was on target, yet no bullet missed any of them....
No one of sound mind could go it and kill like that and be normal in my opinion. 
Imo all bullets were on target - non missed?
The poor twins were killed quite efficiently (thankfully - in a way - whilst they slept).
June was brutally killed (not frenzied) by final shots to the head.
Nevill was shot whilst moving, beaten, and finally brutally killed (not frenzied) with shots to the head.
The reloading was very efficient - although I still suspect other preloaded magazines may have been used.
If the shooter in a frenzied state reloaded from the pile of bullets in the kitchen I would have expected some of the bullets to have been spilled on the kitchen floor and yet the pile of bullets was quite together - if you see what I mean!