She wasn't expecting to be murdered.
I was trying to imagine the outcome. Terri says, "Michael, I just had a Living Will recorded and do not want to be starved or dehydrated ever." And Mike would have responded, "Golly, gee, rats, darn, I guess you are safe."
The problem is that most living wills, advance medical directives, etc. are kept by a family member. So, the likelihood is that Michael would have been the person who had any documents and I have no doubts that he would have destroyed them to suit his desires. For all we know, Terri DID have a living will and Michael just didn't tell anyone and instead chose to "remember" a conversation in which she confided in him alone that she really did want to be killed.