Lets accept your premise that "no evidence exists". Then it is OK that a woman be killed at the request of a husband based on the his unconfirmed impression she would have wanted it.
May I be so outrageous to suggest that a duly elected Representative governing body be afforded the opportunity to inquire as to whether this is good policy?
Perhaps, even in the light of no evidence, we might find other reasons to end unworthy lives like Terri's. Perhaps if someone was once a good Christian who would have never wanted to be a witch (they may very well deny their a witch after becoming one, but what do you expect a witch to say) perhaps we should kill them withouth any evidence as well...just to make sure we respect their wishes and all.
That is not the reality; there were corroborating witnesses as well. Further,
...the right of a patient, who is in an irreversibly comatose and essentially vegetative state to refuse extraordinary life-sustaining measures, may be exercised either by his or her close family members or by a guardian of the person of the patient appointed by the court. (John F. Kennedy Mem'l Hosp. v. Bludworth Florida Supreme Court, 1984)