You know after reviewing the No thing again. It has grown on me. I mean when he looks at luke and says a silent but noticeable No, he looks like he's thinking. (Like him thinking of that night saying No and chopping of Windu's hand and seeing him get fried.) So remebering this, he thinks unintentially saying out loud No again killing the Emperor to save his son from the same fate. That's my 2-Cents.