So if I decided that my spouse was in such bad shape that it would be best to put him/her out of his/her misery by shooting him/her in the head with a gun and causing death that I would not be convicted of murder. Hm.. I wonder why more people don't use this line of defense in their murder trials. Basically I think you are flat out wrong. It would be murder and anyone who takes your advise that it isn't would find themselves serving time. That is except for MS.
So if I decided that my spouse was in such bad shape that it would be best to put him/her out of his/her misery by shooting him/her in the head with a gun and causing death that I would not be convicted of murder. Actually most people who do that are found guilty voluntary manslaughter, not murder. But keep in mind that the death of Terri was lawfully authorized, so it can't be murder or manslaughter.
How do we know it was lawful? Every court in the land (including the US Supreme Court twice) said it was. Give it up.