Here's a question: We don't know exactly what the state of her mind is, but, if you were Terri Schiavo, would you want to live in a half-live, half-dead state where you can observe your hospital room, but not interact in anyway whatsoever with the world? That seems like a hellish nightmare to me.
Suppose the alternative were to be doted over by a couple of loving parents and a warm soft kitty cat (or dog, if you don't like cats), while they helped you receive some therapy to regain some of the skills you'd lost?
To the extent that Terri is miserable, it's because Michael is trying to make her be. There would be no need to "put Terri out of her misery" if we could simply put Michael out of Terri's misery.
"We don't know exactly what the state of her mind is"
Do you think she should to be taken off food and water for weeks to die even if you are right and no one knows exactly what the state of her mind is? What if she is aware of being starved to death, and what if she is aware that she is thirsty and desperately wants water? Would you want to be in her situation?
It's only hellish because Michael Schiavo has denied her flowers, sunshine, tv, pictures and occasionally even the company of her family for the past ten years.