Reminds me of the teddy bear stabbing. Did you ask Julie why she stabbed the teddy bear Brett gave to her as a gift? Imagine that, stabbing a cute little teddy bear!
Jealousy, I should think Alias.
I believe Julie was unable to allow Jeremy to grieve the way he needed to. She being selfish and immature demanded he grieved with her while he found it much easier and less demanding to spend time with Brett as he tried to sort his head out.
Brett was possibly understanding and undemanding whereas Julie wanted chats and cuddles which Jeremy couldn't deal with so she became angry and resentful.
Oh dear, that is NOT the way to a mans heart.