Do you think the only reason Jane really wanted Bree killed was so that the cullens (forgetting about edward's power) wouldnt know about her "partnership" with victoria?

Probably, and also because she wouldn't like to be thought of as a push-over by letting Bree live, also she might have been worried that Bree would tell the Cullens more about her meetings with Victoria, and she wouldn't want them to know that she and the Volturi didn't do anything to stop the newborns attack, after all, the Volturi are meant to be 'good' and enforce the right laws and punishments. If the Cullens found out about their plans and meetings, it would probably make them lose alot of respect for the Volturi, and like Caius says- 'The Volturi don't offer second chances'. So she wouldn't want to give Bree one and go back on what they said. :D