Hypothetically, if, (and this is a big IF) a person happens to injure another person causing them to have an injury which renders them disabled - if that person were to just happen to think that they would get by with it if they happened to manage to have the injured person killed - well, their lack of peace would only just begin, in spades.
Having stated this, I do not know what happened to Terri to cause this condition which she is in. Shiavo is innocent unless proven guilty, of course, so I'm certainly not accusing him of anything.
What happened in 1990 is, and may remain, a mystery even if Terri recovers. The criminal actions by Michael, Felos, and Greer since then, however, are no mystery. There is every reason to believe Terri's condition would almost certainly improve at least somewhat (impossible to tell how much) if Michael weren't preventing it. It would be fair to call the guy a monster for that alone, though of course he's a monster in many other ways as well.