I think from this perspective we are discussing is that she went to the police when she found out Jeremy didn't want to continue it
Oh yes 100 per cent, the only problem Julie had, was how to play her part and past down and it came in the form of help from the Police..........To plan and execute 5 murders and get away with it, is no mean feat, Jeremy needed someone like Julie to Bounce his idea's off, which he did, but once the Honeymoon period was over she became a problem to him.
How to get rid of this person and still carry out the murders, does he get rid of her Pre Murders and risk the prospect of squealing like a Canary to all and Sundry what he was up to, or does he bring her on Board and threaten her as an accessory to Murder and along with the Cheque book fraud and Caravan break in and drugs you'll never become a teacher and you too will be Banged up if you say anything? So he chooses the Latter and when he thinks the Coast is clear and everything gone that would incriminate him..........Julie is then shown the door so he could then live the high life for a while with Brett, something he couldn't do if Julie was in tow?