She also robbed the caravan site with him & told her about the cheque book fraud. Which created a trust regarding crimes.
I believe her WS that she didn't like talk of murder. But seems she didn't discourage him much. So indirectly was in it with him.
I agree, Bamber had invested a lot of confidence in Julie, she was always on board with what he said, but he tired of her as the relationship progressed, he could not rid of her before the murders because Julie would have transpired to friends , nothing to stop her going to Police if he carried it out, she's not with him and not implicated at this stage.
Bamber would naturally think it better once he had done the murders and the heat was off him, who would believe Julie now, Bodies gone, evidence destroyed, Taff on Board, Julie still keeping Schtum but implicated, plan A executed [murders], now for pan B [Julie]