Marty shouldn't be blamed for old Biff changing the timeline in Back to the Future 2 for leaving the DeLorean unattended
Old Biff shouldn't even have changed the timeline in the 1st place. Period. Blaming Marty is like saying Bruce got his parents killed in Batman Begins for asking them to leave the theatre even though Joe Chill shouldn't even have killed Bruce's parents in the 1st place. If someone commits a crime it's the fault of the person who commited it