I still think if he had gone to all the trouble of planning such a complicated and risky crime he would have played her along a bit longer . Keep your enemies closer and all that .
I've never seen Jeremy as a particularly mature and responsible person. Who knows what sort of strains he was feeling -OR Brett's arrival highlighted- in his relationship with Julie? Might he have seen it as entrapment? Did Brett offer a free and 'laddish' existence? I think you're buttoning both Julie and Jeremy into very tight packages -in terms of what they would/should have- in which there's no freedom of movement for either. It doesn't signify, if they do A, that B automatically follows.