If she told people ahead of the murders what Jeremy was saying then he would likely have been reluctant to follow through because he would be the natural suspect and his claims would likely not be believed. Particularly if she had said he was talking about framing Sheila for killing them. If she told people about what he was saying who would believe Sheila did it? Her not talking is what enabled him to go through with it.
Bamber took a bit of risk, assuming Julie didn't mention his plans to anyone, pre massacre. There was always the chance Julie told her mum or a close friend. Even if she thought Bamber was not serious.
He obviously thought Julie was sufficiently under his spell.
He wouldn't be worried about Julie, post massacre telling people how much Bamber resented his situation and relatives. That doesn't make him a killer, besides which Bamber would tell anyone else that would listen the same raw deal story.