Without any other evidence just the facts that JB had an extremely cavalier attitude to criminal activity, a grasping love of being flash with cash, a resentment toward his family and hoped to inherit made the instant and total demise of his family far too convenient.
This^
Also, him calling his girlfriend at 3am to say, 'it's all gone well' or how ever he phrased it. What had, 'all gone well.' Why was it so important to call at 3am ? That's usually only for dire emergencies. He only mentioned there being a problem at the farm at the end of the brief call.
Did he make that phone call from his own house ?
When JB was arrested and heard the evidence against him I wonder what his, 'oh shit, didn't think of that' moments were. I realise he'll never confess.