He was not a cold blooded killer until that fateful night. Even after the murders he said Mcdonald did it.
The relationship did not last long afterwards.
IT DOESN´T MATTER IF JULIE THOUGHT McDONALD HAD DONE IT - SHE COULD STILL HAVE SAVED THE FAMILY.
It lasted a whole MONTH, where she had even seen the dead bodies of them all.
I repeat: IF Jeremy did this, Julie was IN on it. Jeremy would NOT have told her about it unless she was IN on it and AGREED with what he was about to do.