The last time that Gabby was seen alive was August 27th when Laundrie argued with restaurant staff and returned 4 more times to argue more. She was crying and apologized to the staff. It sounds more like Laundrie had anger issues and Gabby paid the price.
While I agree that if Laundrie did indeed murder Gabby it was not pre-meditated but I do think that Laundrie was definitely pre-disposed to murdering her, eventually. Mounting evidence seems to bear that out.