Good point.
One option for Jeremy was to massacre everyone. Then go home & let someone else discover the bodies in the morning.
Quite why someone other than Jeremy would break into WHF & massacre everyone, no one would know. However there is no circumstantial evidence against Jeremy, mysterious phone calls etc.
I'm not trying to help criminals out but here is what one would need to do:
1) stage it as a breakin and steal valuables
2) killing some in bed works but not all because all indicates a panned execution. Kill the most dangerous in bed then call in the others to shoot as they enter or shoot them in the hall so it appears they got up and you had to kill them before escaping because they saw you.
3) make sure you dispose of any valuables unique enough to be tied to the crimes
4) dispose of the clothing and gloves you used as well as the murder weapon
5) pretend you know nothing and tell not a sole what you did
6) if you told someone else you had plans to kill them then you are an idiot and don't even bother trying because you are too stupid to pull it off and will get caught
Do you know how many got caught because they broke my rules? A ton. How many got away but using my plan? Who knows because they didn't get caught.
With Julies testimony the case may still go to court. However the defence could bring up every mad criminal not in jail on the night. Creating some reasonable doubt.
The problem here is if Jeremy was due on the farm first. Putting him at the scene. If he always met or worked with Neville first thing in the morning. People would ask why he did not go looking for him.
If no one else managed to find them first then call for help and say something is wrong they should be up but are not.
It will be obvious they were not killed right before police were called. Ideally the farm secretary woudl find them or Jeremy "naturally" find them with someone else.
Would here be suspicion Jeremy was behind it for the inheritance? Sure but that is not enough to prove guilt and if he followed my steps they would not be able to tie it to him.
Fortunately most people smart enough to get away with murder have no desire to engage in it.