I think Julie Mugford was influenced by several factors which came into play at different stages and I also think that she was carried along by events which she was unable to control. These factors were a need to bolster her self esteem, a desire for revenge, fear and, further down the line, money.
I believe that when jilted by Jeremy Julie told her friend in confidence that Jeremy was responsible for the murders. Her motives at that stage were probably mixed and confused. Possibly she wanted to bolster her self esteem by demonstrating that Jeremy was completely reprehensible and that she had therefore not lost anything of value as a result of the breakdown in their relationship. Having made the allegation it became hard for her to go back on it. It was her friend who made the initial contact with the police so at that point I believe fear must have kicked in as a motive, coupled with a desire for revenge. We do not know the pressure which was brought to bear on Julie by the police, but my guess is that she was told in no uncertain terms that she risked being charged herself with being an accessory to the murders. She was offered immunity from prosecution on all matters, but only on condition that she followed through with her account and gave evidence against Jeremy. There was then no going back. Further down the line the News of the World deal was concluded which gave the additional incentive of money.