I have thought long and hard about this. It is a fact that if SC is not the killer then only JB remains IMO.
He tells the boss that he is fed up with farm work and needs to get out but has no money.
The boss says well kill them. He says I can't do that. so the boss says I will help.
The boss says I have got some sleeping pills you can use, so he tries, but they don't work.
The boss tells JB to shoot them all and a naïve JB agrees, as long as the boss helps him to cover him.
During the murders JB phones the boss and says it is going well. He phones the boss a while later and says that SC is still alive. The boss says shoot her again.
When the police arrive at the farm JB feels uncertain so calls the boss and says I need you here.
The boss arrives, and comforts JB and tells him to stick to his story.
After a while the police begin to suspect that all is not well with the suicide theory. The family are not happy with the police investigation, and start to look into things. The boss realises that the family are on the right track and needs an out. What to do? Then the boss has a brain wave put JB in the frame alone, and save her skin.
The boss goes to the cops and the rest is history.