Jeremy amazingly rang Julie twice before the raid team had entered WHF.
The first time almost certainly before ringing the police. According to Julie to tell her he had 'not slept all night' & 'everything was going well'. The second time to tell her not to go to work & a police car would pick her up. Which was generous of the police as Julie was not a relative.
Jeremy was in a self satisfied mood when meeting Julie. Not confident enough to say he did the massacre, but boashut upl enough to say he arranged it and wise enough to give a proxy who would have been no where near WHF.
There are several reasons why Jeremy told Julie.
Julie would suspect anyway. Jeremy would tell anyone who would listen how much he hated his family. Julie was the person closest to Jeremy prior to the massacre. So would have been on the receiving end of Jeremys rants & unfufilled plans to get rid of everyone.
Julie knew about the caravan break in. Committing a crime against the family, Julie would suspect that Jeremy was also prepared to go one stage further & committ a crime to his family. So even if Jeremy said he had nothing to do with the massacre, Julie would again suspect.
Jeremy trusted Julie enough to involve her in the caravan break in. So he may have trusted her enough to confess an involvement in the massacre.
Julie said herself she was under Jeremys spell. Jeremy would have known this & felt Julie would remain under his spell. If they spilt up weeks or months later, there would be no evidence, even if Julie went to the police.
Jeremy felt Julie would not have the confidence to go through with a confession & trial. She would be scared of getting implicated & they would not believe her anyway. Jeremy had boasted about being 'watertight' & it being an 'open & shut case' with no evidence against him. Julie was only 20 & caught in an unexpected situation.
Jeremy wanted to boast & prove Julie wrong. Julie had dismissed his 10am 'it's now or never' claim. She had also dismissed his previous plans. When interviewed on television she said 'Jeremy liked to say things to shock people'.
Jeremy would have been buzzing & hyped up directly after the massacre. He had lost his immediate family, although intentianally. The closest person to him was now Julie. He had to phone her, even though it was 3.00am. Just to hear her voice & reaffirm that somebody still loved him. After that 3.00am phone call, what he said & how he said it, Jeremy knew there was no point lying to her after that.
Jeremy would have enjoyed boasting to Julie. Once things had blown over even more he would have no doubt hinted to other people of his involvement. He may have even enjoyed police suspecting him, believing they would never have enough evidence to charge him or get a conviction. It has been said Jeremy enjoyed the female attention at the trial.
Jeremy had repeatedly told Julie of his hatred for his family. He may have thought Julie would understand his reasons. Espesically after some nice, expensive meals.
There is some debate on whether Jeremy did jilt Julie. However if they did split up & Julie went to police, Jeremy could claim she is doing it as a scorned woman. Which is exactly what happened, although Jeremy has never said a scorned woman is also likely to tell the truth.
Jeremy had not said to Julie he had committed the massacre. He gave himself a proxy. If the police were informed, the proxy would deny it. And Matthew Macdonald could probably prove he was no where near WHF. If there was no other evidence against Jeremy, - case closed.