Also if it were a hitman why would he want to make it look like suicide? A professional just wouldn't care to mess about like that.
Hi Grahame, my scenario does not imply a professional hit man (with previous experience) working alone, but an accomplice working with JB, and not all plan's go to plan!!
And the suicide plan as planned by JB nearly worked - this way he convinced his warped side kick that they would get some money - eventually 
I wonder why the hitman hasn't ever been found? Especially since he didn't get his cut? This way he could really secure JB's conviction and silence his appeals forever?
Grahame, as previously stated, in my theoretical scenario I did not suggest a professional hit man, but possibly an easily led accomplice working with JB on the promise that he will get some serious money when JB inherits. JB probably had a few very dodgy friends from his cannabis dealing that may have been prepared to go along with his plan.
Was JB particularly charismatic and convincing enough to get another in on his plan - maybe?
The term "hitman" that you keep using clearly has professional implications and a hitman (IMO) would want to work alone - and I do not fully support a hitman working alone.
The Accomplice is clearly not going to come forward to implicate himself, and why would he want to secure JB's conviction and silence his appeals forever? JB' conviction and appeals do not effect the accomplice at all. Unfortunately for the accomplice, he has to do with no payment as the plan (after a month of it going well) eventually failed - Just a thought.
The reason I am promoting this scenario comes about from others posts involving the Brno rifle (or the unique weapon handed in shortly after the murders).
I cannot see SC using two rifles to kill everyone esp. as one was a bolt action rifle, which is even more difficult to use than the semi auto
