The motive itself is clearly money and/or resentment of some kind. However, having a motive doesn't necessarily mean that someone will do something about it.
I think the term "psychopath" is rather meaningless. What is it really? An absence of empathy? A lack of ability to give a toss? That doesn't make someone a murderer.
Jeremy said that his relationship with Julie was breaking down before the murders. I'm not sure about that because he seemed to see her quite a lot. However, if he was getting fed up with her I'm not sure he'd tell her his plans.