If not, what safeguards exist to ensure that such a would-be murderer would not be able to have the state finish off his intended vhctim?
If there exists any reasonable plausibility that Michael's motives stem from sociopathic intent but he is allowed to kill Terri, then it would seem to follow that a sociopathic person in Michael's shoes would be free to do likewise. And to my mind that seems very dangerously wrong.
An interesting supposition. However, one does not make legal arguments on hypothetical questions, but on facts in evidence, and the Schindler family has no such facts in evidence.
If there exists any reasonable plausibility that Michael's motives stem from sociopathic intent but he is allowed to kill Terri, then it would seem to follow that a sociopathic person in Michael's shoes would be free to do likewise.
Unfortunately, you miss the critical point: the law does not operate on assumptions of "reasonable plausibility," it operates on facts in evidence. The facts to back up your hypothesis are not in evidence.
Go back and reread the posts in the Save Terri saga. Every reversal is explained away in terms of ever-larger conspiracy theories that eventually encompass most of Pinella County's population. It's never explained away in terms of "the parents are arguing from a very weak legal position without the very strong evidence needed to successfully do so." Indeed, much of the evidence the Schindlers have presented undercuts their own position.
"When the law is on your side, argue the law." (Michael Schiavo's main tactic.)
"When the facts are on your side, argue the facts." (Schiavo's secondary tactic, used when the Schindlers do something really boneheaded, like seeking "treatment" from quacks and frauds.)
"When neither the facts nor the law are on your side, pound the table and hope the judge is a bigger fool than you are." (The Schindlers' main strategy, and a woefully ineffective one.)