What I can say with certitude is that at Pinellas Park at the time of the killing of Terri, we saw detractors who could have been twins of Dr. Linda, very vile detractors at that, so have no trouble seeing this as a highly likely real letter. We also saw first hand the results of the Crist actions.
I notice also that many Crist supporters use the same tactics of the left, particularly projection, in which they visit upon their adversaries the very tendencies they, themselves, sport. It is much like the anti-gun folks accusing us of being potentially violent because they, themselves would be violent if the guns were in their own hands.
Anonymous letters are always questionable.