The actual comission. It is a daunting task to climb into a dark house in the middle of the night, EXPECTING to be able to control five people all on your own. You cannot be sure that everyone is in their beds. Elderly people usually get up during the night to go to the bathroom for one. Dogs barking waking everyone up is another risk factor.
I have heard some attempts of explaining where Sheila was while the rest of the family were being killed, but none of them sound plausible to me. She wasn´t "led" to the bedroom by Jeremy - it is BS, somebody must have gone directly to her room to control her! She wouldn´t have been passive while her boys were being shot - even Colin says so.
If I am to believe Jeremy did it - and I am quite willing to - there must have been an accomplice. Save yourself the trouble of trying to convince me otherwise, won´t work, I can´t see Jeremy doing this alone.
The whole reason to do it at the hour in question is in part because they will be easier to manage because they are in their beds. The chance of any being up at 2:30 is unlikely let alone more than 1 up.
There are numerous cases where single killers have slaughtered entire families overnight. There ar ealso cases where they salughtered some an d fled and the remaining members of the house di dnot wake up and only found out in he morning what happened.
Not all such cases involve guns. There are cases of knives and axes being used to kill everyone.
Just recently someone in California who went on a rampage with guns killed his 3 roomates using a knife.
The suggestion it is not possible for one person to do it is not true.
It would be safer to use an accomplice but on the other hand that leaves an extra person to talk and who needs an alibi.
If he hid the keys instead of leaving them in the locks and replaced them after the murders tha towuld make it harder for anyone to get out of the house before he killed them even if they did manage to reach a door. Whether he did that or not who knows.
He might not have bothered expecting them to be killed in bed.