I have said this before,but I do not believe that Jeremy pulled the trigger on that fateful night.
To plan the murders in advance,carry them out and then be so calm in the aftermath,would,in my opinion make him a pyschopath.
Also,I put alot of weight on PC Mercer and his dog.The german shepherd is said to have approached Jeremy but gave no positive response that Jeremy had fired any weapon.for which it was trained to do.
However,I am not fully convinced that Sheila was responsible either.
For me,it boils down to this - If Nevill was not in bed when it all started,then I can believe that Sheila was responsible.
If Nevill was in bed with June when it all started,then logic tells me that Jeremy is indeed guilty.