You fail to quote that correctly. Mr. Schiavo's statement was cut off by Mr. King's interruption. (Here's the transcript: http://transcripts.cnn.com/TRANSCRIPTS/0503/18/lkl.01.html)
Just like I might not know exactly what my friend would want for lunch, I can still make a decision and not bring liver and onions. Even if someone doesn't know my exact wishes, they can be the best one to know the general parameters of my decision process and be the best to decide for me.
That doesn't jibe with these claims that he was following her wishes. He did what HE wanted. He had her killed. Period.